Just to note: I believe the majority of difficulities people have with broadband phone service can be linked either to (a) the equipment or, (b) their broadband service.
While the latter is probably more common, it is a clear fact that the 'box' you hook up makes a big difference as well. Carriers such as Vonage, Packet8, etc. offer, or have offered, different models from Linksys, Motorola, etc. Which you have/get may make a significant difference. I, for one, can tell you that my experience has been that the Motorola equipment for Vonage is superior to the Linksys box, which, unfortunately, is the more often sold/packaged product.
Regardless, the point stands that the equipment can make a difference, even utilizing the same service. Still, I believe a greater factor is your bandwidth and quality of your broadband connection.
For the record, I have had less voice quality problems with my Vonage service than I had with Sprint/Embarq. The vast majority of the time I don't even consider that I have anything other than a typical phone line... oh, except when I pay the bill, of course.
