I bought 2k11 for $15 used at gamestop a few months ago. I had played it with friends on xbox last year at college, but didn't buy it right away.
I'm glad I bought it when I did, and glad that I didn't get 2k12. My same friend got 2k12, and we played recently at his house. It just feels different overall, in a bad way. It felt to me like the game is trying to do too much all at once, and it feels more disjointed than a basketball game should. I mean with the new post moves and all the new animations, you'd think 2k12 would be way more fluid. But it is just too sluggish and deliberate at times; it just doesn't feel like basketball to me.
But 2k11 is obviously less complex I guess you can say; it feels more fluid and like real playing real ball. And you just can't beat the Jordan challenge. I got back into it today and I absolutely love it.