Vonage fan here.
I have two phone lines into the house (both served off the same vonage box). I wired it directly into the house wiring -- so all of the jacks in the house connect to the vonage box (if I hadn't told my family members we used Vonage, they wouldn't know).
Contrary to the experiences of many others.. I have 3 tivo's -- none of them have ever had any trouble dialing out. I also use it heavily for faxing. No problems there either.
911 isn't an issue with Vonage -- they let you set up the 911 service for each line.
The line quality is exceptional. I also do at least 5 hours a week of international calls (a huge motivation in my original switch to Vonage). I also LOVE all the extra features they give you (free) -- such as the ability to have your Vonage number ring your cell phone AT THE SAME TIME (I no longer give people my cell number. I give them my vonage number. If I'm home, I pick up my landline -- if I'm out, I answer my cell).
Bottom line: I will never switch back.
If you DO decide to switch to Vonage, let someone on here know as they have a great referral program (2 months free!)
Brian