I have a K2i, and I won't be upgrading until either it breaks, or a new Kindle comes out that supports ePub and library books. That'll be at least K4, so maybe color screens will be standard by then. I'm not craving for color that much, but I wouldn't mind color, either.
Since I have absolutely no problem with K2's contrast, the only thing that really seems like an upgrade with the K3 is the doubled battery life. The extra storage is nice, too, but not really a factor for me since I doubt I'll ever come close to filling the K2's storage.