Mine's fine. There's slight backlight unevenness right in the corners (only really noticable when the screen is black), but other than that, if there's any yellowing it's so slight I can't actually see it (after much experimenting, I think it MIGHT be slightly more yellow in the middle, but I'm not actually sure!).
The colour and everything else are about what you'd expect from an LCD, not perfect, but close enough (and way way better than the cheap 19" I run along side it!)
The 20" apparently has a cheap TN screen (same as my 19"), which would be fine so long as you're not doing graphic work that needs decent colour.