I've been with Vonage for about three years now and have had overall excellent results. Comcast offers a similar program bundled with their CATV and/or Internet services, but once you get past the introductory price its not worth it.

While I appreciate "one-stop shopping" convenience, I prefer options and choices. If I have to get VoIP service from my broadband ISP, I may drop any landline and use my iPhone as my only personal phone. A pity, because I like having a personal fax line, and I'm not yet a fan of Internet faxing.