We have had Vonage since August. It didn't actually start working until at least October, because when we wanted to cancel in September, we were told that we would have to pay a $200 cancellation fee because we were past the 30 day trial period, and it didn't work for a full month after that. I second their horrible customer service. At one point when we were trying to get our line to work in the first couple months, DH finally told them, "You know, I have been on my cell phone with you for an hour and a half, away from my family, and I'm paying out the nose for overage charges for this call and every other one that is getting forwarded from our Vonage account because it's still not working. I'm hanging up."
They finally sent us a brand-new wireless Vonage device, and we haven't had too many problems since then, but those first months were enough to sour me. When our contract runs out, I'm not sure what we'll do. We also have cable internet, and when the signal goes out, so does the phone. That worries me a little, because we don't have cell phones.
On the plus side, it is very cheap, but you do have to consider the fact that you need high-speed internet, and it is a 2-year contract, so you'll need high-speed for two years.