Mostly an old thread (2007), but FWIW in the interim I've found that I tend to carry rain chaps a LOT more than I ever tend to wear them. One nice thing about chaps is that they're easier to put on and take off than rain pants, but handlebar's comment about a rain skirt is right on --- even easier to put on and take off, and more breathable.
I suspect that if you're in a situation with a lot of wind that you would prefer the chaps, but for most situations I think I like the idea of the skirt better (of course, those that use ponchos effectively have a skirt built in).
I just bought a cuben rain skirt from zpacks.com, very very light, so kind of a no-brainer to carry on trips where I'm not using a poncho. I suspect this will become my more common lower-body raingear in future, though for trips around home ... well, a rain skirt looks a little odd to wear walking to the store or something like that!
Of course, rain chaps look pretty deviant too if your rain jacket doesn't go well below your hips.