There are tons of foreclosures in all areas, including nice areas like Beverly Hills and Manhattan, etc. Economy died so tons of incomes went down and lots of people couldn't afford it anymore.
Compared to regular sales, the major "downside" is that the bank owned stuff generally is in worse condition than regular sales. But it's not always the case. It's just that when a bank owns something, they're lazy and don't care to fix it up and they take forever to sell it. So your cost to fix it up might be higher, but you're generally buying it at a lower price.
I wouldn't be surprised if foreclosures tend to be crappier than normal sales, but think of it this way. If you're buying to live, you only need 1 house. So just filter out all the crap in the areas you don't like and search for the areas that you do like. Even if there are 1000s houses in crappy areas, you only need 1 to be a good fit for you to make it work.
Regarding home values, yeah, Detroit lost more value than Manhattan, but if you're buying below market on these depressed prices, then it doesn't matter as much. So, if the house used to be worth 200k, and you buy it for 100k now, it's not that big a deal unless it goes down to 50k. Just don't buy something because it's "cheap" compared to the price at its peak. Still make sure it's good value.