Interesting this thread should pop back up as I've learned a great deal more about Spring Valley since posting it. Here is what I have learned.
East of the 125 but not in the Hills, Spring Valley ranges from extremely trashy to gang-bang dangerous to somewhat liveable yet totally run down neighborhoods. Things change from street to street and from house to house. Everyone has bars on their windows. When you drive past the people they look at you like they want to kill you.
In the hills of Spring Valley there are a lot of older homes but the neighborhoods feel about the same as the older parts of Santee. Not yet run-down but obviously older and declining. Few bars and the people seem ok.
West of the 125 (about a mile deep) there is a small strip of land considered to be Spring Valley just before it becomes Encanto. I have found the nicest neighborhoods to be in this area; between Paradise Valley Road to the south and and Troy/Palm to the North. The houses don't have bars on the windows, are generally acceptable in appearances, the neighborhoods are tidy, and when you drive past the people they seem indifferent. The houses are clearly older but most people seem to be maintaining the paint/years/roofs and things look nice enough. It isn't great, but this area is significantly nicer than the majority of Spring Valley to the west of 125.
The primary shopping area is along the East side of the 125 between Paradise Valley and Jamacha. It's not a great place but it's good enough.
I have also found that Spring Valley has very poor soil for building. The majority of homes in the valley have cracks in their foundations. I have looked at 10 homes currently on the market that have severe failure in the foundations, causing giant upheavals and tearing the walls from the ceiling. A "good" home in the valley has only small foundation cracks with minor slopes in the flooring. Anything in the MLS in Spring Valley that is "cheap", is priced such because you're going to find a giant rip down the center of the house; often so bad that you stumble to walk around.
Also check
National Sex Offender Registry whenever considering a neighborhood. Spring Valley has child molestors all over the place. Live close to elementary schools to stay away from these monsters.
In summary, stay out of the valley part of Spring Valley and things are OK. I wouldn't say any part of it is "nice". But there are areas that restore your hope in humanity and make you think your neighbors might actually smile and talk to you on occasion. It doesn't seem like any part of this area is being considered for rehabilitation so I expect to see Spring Valley to continue to decline and wouldn't be surprised if much of the area is eventually labeled blight.