I have been on Vonage on a cable broadband connection (Time Warner Cable company and Earthlink ISP) for about 6 weeks and I am personally basically satisfied, although I have already had several outages lasting over an hour. I know this was a problem with my ISP at least once and probably the other times as well. I am satisfied with it if the outages become more rare. My wife is a stay-at-home mom who lives with the phone to her ear and she expects 100% reliability. When there is no dial tone, it becomes my problem. If I were single, I wouldn't care. We will cancel if the outages are more frequent than a few per year, and I am pretty sure they will be. I just don't need the hassle of my wife complaining about it when it is down. There is more to go wrong with VoIP than landline because an outage with any one of the following will knock out your service: Vonage, ISP, Cable, electricity. The phone company is more reliable.
The sound quality is generally on par with land line, but not 100% of the time. You will notice occasional minor problems with connections or noise, etc., but nothing I find annoying. It is fine in my opinion.
911 is not quite up to par in most areas in that the operator will not see your name/address/phone number in most cases and your call will probably be routed to a general number at your local police station rather than the emergency 911 call center. Vonage is supposedly upgrading to a true 911 system (E911) nationwide gradually. Some VoIP companies have no 911 service at all and at least one has more E911 than Vonage and this may be the biggest distinction between the various companies.
All in all, it is a good way to save money, but you are compromising on the service reliability/quality somewhat vs. the local phone company. Since most people have a cell phone to use when necessary, this is probably not a major problem for most people. Or it would be ideal as a second phone line. We are paying less money for both the VoIP and a broadband connection than we were paying for a similar unlimited calling plan with the pnone company before. So we basically get free broadband and save a little money, too. And no longer have the expense of the dialup ISP account. The unlimited plan on Vonage is $25, really about 27.50 with taxes and fees. There are competitors pricing at about $20 and I would check them out first.
Concerning number portability, keep this in mind: If you want to port your current phone number to Vonage, etc. then you should not cancel your phone service. Request a port through Vonage and let them cancel it for you. I have heard complaints that Vonage can be really slow getting this completed. And if you ever want to port your number away from Vonage, they reportedly will not allow it. Their position is that they do not have to because they are not governed by the FCC, not being a "telephone company" in the technical sense. So you may lose your phone number if you decide to dump Vonage (or whatever VoIP service you use). This does not seem like a very customer-friendly policy to me.
A lot of people do not know how easy it is to route the VoIP service into their home's telephone wiring network so that all of your house's phone jacks are active and you can use all of your existing telephone equipment as usual. You basically just disconnect the phone company from your house at the junction box outside your house, then use a standard phone cord to plug your phone adapter into a wall jack. This will work in most cases depending on some variables such as the kind of phone adapter you have and your existing total ringer equivalency number, etc. There is more detail searchable on the net.
If you choose not to wire VoIP through your home wiring and you leave the phone company connected, I found in my case that even though I disconnected my phone company service, I was still able to pick up the phone and get a dial tone from the phone company. I could not place a normal call, but I could call 911. This can be a way to overcome the 911 limitations of VoIP.
In summary, I would say to give it a try. Especially if you are not a real stickler for 100% perfect service reliability and/or you have a very stable broadband connection. Maybe even keep your old phone service while trying out VoIP so you can fall back easily.
And finally, if you do sign up with VoIP, see if you can get a referral bonus for a fourm member. I know Vonage has a referral program, and probably others do too. It's free money for someone.