One of the many hats I wear happens to be a Vonage Dealer and end user myself (3 lines and 1 dedicated fax).
Vonage works well for businesses as long as you understand its limitations and yours.
Vonage requires 110-128k of upload bandwith per line active at any one time. www.vonage.com recommends 90k, but at the 110+ range the calls are crystal clear. If you have 2-3 phones all going at the same time, you need to have the bandwidth available for each line.
Your "typical" home (read lite, express, etc.) DSL or Cable service will cap out around 128k upload. You will need to check with whatever service provider you are or will be using and make sure you are getting the upload speeds that you will need.