I have heard some flaky things about the old ones, but you can't get those any more anyway. I haven't heard anything bad about the new ones. (anything that comes with an ADC connector) I have heard that they scratch kind of easily if you knock one over and drag it face down across the corner of a table. Although I have trouble believing Apple's monitors are unique in that regard.
Anyway, I haven't heard anything noteworthy about Apple's latest batch of flat panels. This means that they're not so bad that you notice immediately anyway. They may be great, long lasting screens, but they just haven't been around long enough to tell. I'll probably get a 17" one soon, and another larger one in a year or so. I'd already have one but I decided to get a new car instead. Saving up money takes so much longer when you have a car payment to make. At least my rent is cheap. :-)