SingleNurseMom, I'm in Shingle Springs, and the Sacto forum is where I hang out - many of us who live up here in the foothills work in Sacto or go down there for "big" shopping...so you're in the right place
Well, I just wrote my master's thesis about all of these places, so hopefully I can help. My opinions of course, will differ from others.
Where are you coming from? Are you looking for town living, or "country" living? Are you comfortable driving winding roads with no shoulders? How old are your kids? Will they get bored living in the "country"? I'll give you some general information, but knowing some of these things would help me give you a more precise answer. It's hard to say where the "best" place to live is, it depends on what you want and need. There are a lot of different living conditions up here.
For your general question, "How hot does it get in the summer? (As bad as Sac?)": Well, yes and no. On my yahoo personal page, I keep the temps of Placerville, Shingle Springs, and Sacto up so I can see the difference. Today, for example, Sacto is 92 degrees, Shingle Springs is 98 degrees, and Placerville is 94 degrees. Usually, it's 10 degrees cooler as you go up 1000' in elevation. Pollock Pines, today, is 92. I worked at my daughter's youth group's fundraising fireworks booth in Sacto on July 4th, and I guarantee you that it is much more humid there, than in the foothills (by humid, it is NOT east-coast humid, by any means!). Sacto tends to cool off more at night because of the "Delta Breeze", which we sometimes can feel up here in the foothills. That said, I'm from SoCal, and the week or so of 100+ temps in the summer doesn't absolutely kill me. I don't like it, but I can live with it for the weather for the rest of the year that I love.
Schools - for public schools, Buckeye School District (serving Shingle Springs, parts of Cameron Park, and parts of El Dorado Hills) is highly recommended. We were very happy with Buckeye Elementary School - it is like a country school, teachers from various grades, and principal and staff, knew my kid, and not because she was bad

. Camerado Springs Middle School, we didn't like so much. Great administrators, but the teachers can be tough and unforgiving, WAY too much homework (and I'm a former teacher). It is highly rated, though, but if your kid has any learning challenges, or is even just "average", expect that you could have headaches. I've heard the Buckeye District is switching over to a modified calendar this year, with like 6 weeks off between Thanksgiving and New Years. Could be a headache if you have kids in high school on a different schedule. I haven't heard of any "bad" school districts up here, but there are -ahem- certain demographics that lend themselves to some kids not being supervised by parents and they tend to get into trouble. I don't think I'd personally live in Pollock Pines for this reason, with a school-aged child. You can't always go by test scores up here, there are pockets of lower socio-economics up here, and that can lower test scores, but doesn't mean the school is "bad". Personally, I'd much rather have my kid go to a school in the three-school Motherlode Union School District, than have them go to any school in "upscale" El Dorado Hills, where the kids are quite wealthy. Regarding the Montessori schools: I only know of the California Montessori Project in Shingle Springs, which is K-8. I interviewed there for a teaching position, and it looks like a great school. You asked about charter high schools, I only know of Shenandoah High School, which is on the campus of the newer Union Mine High School. From what I understand, people are happy with it - it is more like independent learning.
Okay, on to individual communities:
Placerville: The only incorporated city on the west slope of the Sierra in El Dorado County. Lots of history, charming, thriving downtown (take that, Walmart!!). I don't know Placerville Union School District (serving elementary/middle schools), but I do know staff within the Motherlode School District, and they are very caring people. Homes range from historic, in-town (near Marshall Medical Center) to horse property on the fringes. I work in Placerville - it gets snow a couple of times a year, generally - and depending on your elevation, sometimes a lot, sometimes it melts off that day. Newtown, I believe, is an area adjacent to Placerville. You will likely want to stay away from the "Upper Broadway" area of Placerville to live (although one of my favorite Mexican restaurants, Tijuana Taqueria, is up there). The "food kitchen" is up there, some neighborhoods look sketchy, and there are some transient people around. I don't feel threatened there, but wouldn't want to live there. Oh - some winding roads, depending on where you live.
Shingle Springs: I love Shingle Springs. Not much left of its old center, it ranges from expensive horse ranches way south of highway 50, to 5 acre lots with gorgeous canyon views, to homes surrounding Holiday Lake, to 1/3 acre lots near Ponderosa High School. It feels very "country" in Shingle Springs, even in the "denser" areas. Pros - the country feel, the Train Stop Cafe (best breakfast in the area!), Buckeye Elementary. Cons - the traffic generated on Ponderosa Road at the beginning and end of class for the high school. Shingle Springs has gotten snow twice in the past 5 years. Depending on if you're close to the freeway or not, about 8 miles from Placerville.
Cameron Park: Right next to Shingle Springs, more suburban in feel, more tract homes (although still a large share of custom homes). Several tracts are adjacent to the library, the middle school, Blue Oak Elementary, and the new community center (this could be a plus; you should note that most areas of the county are not conducive to walking or riding your bike anywhere safely, except for this one area). There is a lake with swimming, you can rent canoes there, great fireworks on July 4th. Cons - there are some apartment areas that are less than desirable, the shopping area is mixmatch-looking. Rarely ever gets snow. You are probably 10-15 miles from Placerville.
Pollock Pines: If you like snow, Pollock Pines ALWAYS has snow! Keep in mind, that means shoveling or snowblowing before going to work! You are out of the oak/scrub of Cameron Park/Shingle Springs, and higher than the montane woodland of Placerville, you are in the pines! Fresh. Jenkinson Lake with its Sly Park is a great place to fish/swim/boat/picnic/camp. There are some undesirable areas along Pony Express Trail that you may not want to live near. As I stated earlier, some of the kids can get into trouble (not all, by any means, but some school bus drivers prefer to NOT drive the Pollock Pines students!!). I would think, depending how far you are from highway 50, you are about 20 minutes away from your work in Placerville.
Camino/Cedar Grove: Right next to each other. Definitely gets snow! This is the "Apple Hill" area. Lots of orchards (apple, pear, stone fruit), berry picking, pumpkin patches, Christmas tree farms. Mixed forest - oaks, pines, cedars. HEAVY traffic from Labor Day until Halloween - lots of craft shows, people enjoying fall harvest festivals at the various farms. This area is between Pollock Pines and Placerville.
Snowline Camp: Have no idea where this is!!
Pleasant Valley: It's, well, pleasant! A very pretty valley, extending from Diamond Springs (outside of Placerville) then up towards Pollock Pines/ Jenkinson Lake. Some agricultural districts in this area, very country. The closer to Jenkinson Lake you get, the more remote it gets. Gets snow up towards the eastern end. Close to your work in Placerville - depends on how far out you get in the country - parts of Pleasant Valley are relatively close to Placerville.
Diamond Springs/El Dorado: Has some of its remaining historical buildings. These towns are right next to each other. Some very nice areas, some areas I would avoid. I know friends in El Dorado that go through a not-great area, to get to their home overlooking a canyon. I also knew people that lived on Pearl Place - don't go there. Union Mine High School/Shenandoah High School, the newest school in the high school district, is located here. In the Motherlode School District for elementary/middle school, what I consider a plus. So, there is in-town living, and some very rural living as well. Snow - not much. Close to Placerville.
Somerset: The Somerset/Fairplay area is turning into the foothill "wine country", with many vineyards. Very rural. Winding roads, country schools. I would say, in easy driving, 30 minutes to Placerville (? my guess).
Gold Hill, Coloma, Lotus: Very pretty areas. Some winding roads. Definitely in the country. Good 2-school (k-3 and 4-8) district. Gold Hill is unique in its temps, definitely mild (no snow) and perfect for citrus crops, and olives! Coloma and Lotus are very country, oriented to the river rafting/kayaking industries, with Highway 49 through Coloma going throught the State Park. Homes can be custom, or historic. Gold Hill is closer to Placerville, probably 20 minutes or so. Coloma/Lotus is farther, probably 30 minutes or so. No snow in these places.
If you are more of a small town person, not upscale, I recommend that you stay away from El Dorado Hills. People in the rest of the county tend to not think kindly of the more upscale, living in gated neighboods, EDH types. I'm not kidding, people from other parts of the county won't associate with EDH people, and vice versa.
When I moved up here, my real estate agents didn't explain where we were driving to, what the communities were like, and I was always feeling lost. They were insistent we buy in El Dorado Hills, which with its CC&Rs, very strict, we could see we wouldn't like. We fired them, and found an agent that after a couple of houses, was in tune with what we liked. So check out all of the areas before you decide to buy, if you will buy. Good news, if you're a buyer, prices have come way down since last year.
Let me know if I can give you any more pointers. The best way, though, it to come spend a weekend and drive through all these towns.