I have a friend that uses it, and it is usually fine unless they are using the internet and trying to talk on the phone or downloading something then it is super annoying to try to talk to him. He has to kick the kids off the computer to make long calls.
So if you will use your internet and talk on the phone at the same time, skip it unless you have fios then you might be ok but with any other internet provider, your bandwidth would prob mean you need to be OFF the internet for good phone service.
It is a huge money saver, so if you can make it work, great! Might be worth some compromise.