i've got teleblend (formerly sunrocket, well, sort of). Its VOIP just like vonage, just much cheaper ( I think I pay $12 a month). Other side is I think there is more set up involved on my end.
Anyway, I like it. It suits my purposes perfectly. I'm mainly a cell phone user, and just have a land line for the hell of it, and for my alarm system to work over (even though it has a cellular uplink as well)
It was kinda crappy when I used DSL, and was using most of the bandwidth online (streaming video, etc.), it would sound choppy, like you had a bad cell connection almost. Its fine now that I have cable. I can surf and place calls clearly.
Only other negative is it won't work if you're power is out. Or if you're internet is down (which seems to be the more common thing)