I have some 200 weight fleece pants from Campmor. They make a pretty reasonable quality item for a great price, at least compared to things from North Face, EMS, REI, etc. They use Malden Mills Polarfleece, so it's not some cheap imitation, just a less expansive brand name! I find that I wear them under shell pants in wet/snowy weather. They are light, provide great insulation, and let perspiration flow through.

The positives have already been mentioned. I think the only real negative is if you're wearing them around a fire... They don't handle embers very well. You may end up with a few ventilation holes if you're not cautious around the fire. If you wear some kind of more durable shell over them, they are fine.