For those that don't know, Vonage is a phone line through your high-speed internet connection. Although it works with DSL, you have to keep your land phone line in order to have DSL, so Vonage works best for those with cable internet service, in my opinion. I've had Vonage for over a year now (through my cable modem, natch), and I've had zero problems with it. Of course, the one time my internet was out, I did not have phone service, but that's how it works. I get unlimited phone calls throughout the U.S. and Canada, voicemail, caller id, call forwarding, can check voicemail online, call waiting, 3-way calling... and tons more stuff for less than I would pay with any land line phone company. Furthermore, the call quality is just as clear as with a land phone line. I can't accurately rate the customer service since I've never had to use it or the geographic coverage since I only know I'm covered. Also, you can take it anywhere in the nation with you, hook it up to a high-speed internet connection, and call anywhere in the nation for free, just as if you were home!
I'll also add that looking at the website that they have added lots of plans depending on how many minutes you want to buy and where you call most often (Africa? They have a plan for you!). The equipment you hook up to your internet connection is free. In most areas, they also now have a feature where it will give 911 your home address when you call emergency services (as this was a concern early on); you just sign up for this feature and type in the address you most often will be using this product from.
I'll admit I was skeptical at first because the review were inconsistent. However, Vonage couldn't have been a better choice for me. I would recommend you have a cell phone just in case your internet connection goes out, but that's just a paranoia of mine considering my land line phone service went out more often than my Vonage (again, it's not Vonage, it's my cable company's internet connection) goes out. Since they're still offering one month free (two months if someone refers you - the person who refers you also gets two months free), what have you got to lose?