I actually use Vonage. Curcuit city was running a promotion where, if you signed up for a contract with Vonage they'd give you a $200 gift certificate. Since my business phone was about 10 bucks a month more expensive anyway, i switched.

Seems to work pretty well except for the usual problems, if the net is down your phone is down, plus it takes up a slot on your router, which may or may not be a big deal. Otherwise they are pretty much the same as regular phone (except long distance is free), so I dont see why not to switch.