It's true I don't know a great deal about monitor technology, however, it can't be denied that the U2412m has a higher response time than many other monitors available and it is 60hz which is apparently less preferable than 120hz.
In all other respects I think it is a SUPERB monitor. I've not noticed any issues with backlight glare like others have mentioned (sounds like that may be model specific) It's just those crucial milliseconds that mean life or death in fps gaming that are so critical.
Maybe the U2312hm is the one to go for with it's reported 0.6ms response.
However... the tried and tested 120hz TN panels really seem preferable for gaming to me. If only you could have the best of both in one screen. If I can afford it I might keep the Dell and run a TN panel for gaming as well.