I mostly liked Vonage. It worked well enough, although we would occassionally have it go down on us on weekend. We had it for about 2 years. I installed the Vonage VOIP device behind my router so it didn't affect the latency at all.
I changed to AT&T CallVantage (Vonage left me the VOIP box and didn't want it back) - mostly the change was due to the problems with flaky weekend connections, but also we got a good deal for switching. CallVantage requires the VOIP box between the cable modem and the router, but my latency didn't change when I installed it.