If you think Vonage is going to be a big player long term in VOIP.. BUY
If you think Vonage is going to get buried by ATT, Skype, Verizon, Sprint, etc.. Don't buy
I'm thinking Vonage is not going to live up to expectations since you can now get similar things from the companies people know.
Also the POTS companies are waking up a little bit. My in-laws are FINALLY dropping dial up because Sprint is offering them unlimited domestic POTS and ADSL for $70 a month. About the same price as cable broadband + Vonage. Bottom line is that Vonage's previous competitive advnatage (Price) is less of a factor or no factor. And their service is inferior to POTS IMO.
Note. I'm a former Vonage subscriber now using a combination of cellular and Skype for voice. In my case the price of Vonage was too high and the service redundant YMMV