I've used a Sony Trinitron 21" flat screen for years. I recently upgraded to an 18" LCD and then a 20" LCD (had to give the 18" away). I've gotta say I still like the Sony the best. Wyrd pointed out the problem with games - the LCD is fixed-pixel and the dark areas aren't quite as good. Also, it's locked at a slower refresh rate which took some getting used to. I will say that Text on the LCD seems a lot sharper, which helps with coding. But that flicker (which isn't really a flicker, but seems that way - hard to describe) can be a downside as well.
As it is, the 18" LCD from Dell was about $100 cheaper than the 21" flat Sony (which is an AWESOME monitor). The 20" was about $450-$500 more but you actually get more viewable space (but just a bit). A friend of mine is trying to get two 18" LCDs (he's got an 18" LCD and old 15" rounded monitor for now) - I'm crossing my fingers that he won't get them, I'd be too jealous
Of course, there's also desk space to consider. That LCD is VERY nice for leaving some extra room in front, if you don't have/like keyboard trays. I like being able to move the keyboard forward and have enough room to eat... I mean put a notebook for writing down important work "stuff".
-ner
PS Because an LCD is pixel-locked, make sure you like the resolution it should be running at. The 15" is 1024x768 (I think), 18" is 1280x1024, and the 20" is 1600x1280 (I think). You can't go ANY higher, but you can go lower, As mentioned, going lower will cause the monitor to do some doubling of some pixels, enough to make certain things just a tad bit "off", usually fonts or other things that have straight lines.