Here's my take from my limited time in Boise (a week in mid-August '11). I retired to Huntsville, AL from SoCal 2 years ago and have had a terrible time dealing with the AL climate (tornadoes, high humidity, record heat, torrential rains, etc.).
The week I spent in Boise, the temp's were right at historical averages: highs in the low 90s and nighttime temps in low 60s. I could not believe how comfortable I was spending time outside during the day (something I've had to give up here in AL). There was usually a breeze, and that combined with the low humidity (around 20% vs. 50%+ in AL) made life so much more pleasant! My mood was so much better. It was also AWESOME to be able to sleep with the window open and feel a cool breeze!
2) cold/snow: my research indicates 19"/yr. of snow is historical avg. (20 "snow days") and that avg. highs/lows in Dec. and Jan. are 37/24 F. Spokane far snowier: 44" on avg. and 34 snow days, with similar temps.
3) In January '11, there were 14 sunny or mostly sunny days. Same in February. I've read that snow often melts by midday because of the sunshine and highs in the mid 30s. One website shows Boise with 2,994 "sunshine hours" per year vs. 2,578 for Spokane and 2,174 for Seattle.
Boise (elevation 2704') is arid and reminds me of high desert country in CA. I've never been to Spokane (elevation 1843'), but the few photos I've seen seem to show more greenery. Spokane gets on average 16.7" of rain annually vs. 12" for Boise.
I hope locals can add more about the winters. I fear they would be too cold and snowy for me. But if the summers are like what I experienced a couple of weeks ago, they are great!