We've had Vonage for quite a few years. I love the featureset (true voicemail with e-mail alerts, amazingly flexible call forwarding and "simul-ring" options, etc.), but we've been considering dumping it.

Downsides to Vonage:
1) Call quality: If you do any serious UPloading you're going to run into problems with poor outgoing call quality (i.e., people won't be able to understand you). The sound quality on our Vonage line was great until we started running online backup services on our computers. Now whenever mrsmicah calls me from the Vonage line, I can always tell if the computer is backing up because I can't understand her. I've tried playing a bit with QoS on my router, but haven't yet managed to solve it. Incoming quality tends to be less of a problem since most home broadband connections are asymmetric. For example, my cable modem provides 10Mbps downstream but only 800kbps upstream, so I'd have to be downloading a lot of stuff from some VERY fast servers to max out my downstream pipe, while all it takes is a Mozy session to max out my upstream.
2) Reliability: As mentioned, any VoIP service is going to be dependent on your power and internet connection being up and running. Our power and internet are both very reliable, so it's not been much of an issue. But in a power outage, it definitely won't work.
3) Price creep: We have the 500 minute plan, which used to be $15 plus a dollar or two in taxes/fees. Every few months they seem to tack on some new fee or raise the price a little, though, so I think we're now paying $24 all-in. It's still not that expensive, but it's a huge increase percentage-wise, and as RichardInSF mentioned, you can get many of the same capabilities using Skype for much, much less.

Some other responses to thoughts in this thread:

• BettyKat: you'll want to read the Verizon FiOS vs. Time Warner Cable thread from June. To my knowledge TWC, FiOS and RCN are the only cable TV providers in most of Manhattan.
• Note that Verizon isn't the only non-VOIP provider out there. Even though RCN is a cable company, for example, their phone service is POTS (plain old telephone service) not VoIP. I'm not sure if they use their own physical copper line into your apartment or if they simply resell Verizon's line, but it's definitely POTS.
• If you get Verizon FiOS you may not be able to get (or keep) a Verizon POTS phone line. I was told by more than one FiOS rep that they rip out any existing Verizon copper line when they install the fiber optic line for FiOS, although if you have phone service from another company (e.g., RCN) they will leave the copper in place.