We've done it, we've switched to Vonage (well, Qwest gets us for another 2-3 weeks until the phone number switches over). Vonage finally got numbers local to Corvallis, so we could keep our number. And now we'll no longer be gouged by Qwest for nearly 3X what Vonage charges.
And, it was super easy to install, just as easy as the Mac mini. I plugged it in (got the phone cord in the wrong port at first - and the phone gladly told me I did it wrong), and I could make calls. Vonage has all the features I'd ever want: voice mail, call waiting, call forwarding, completely configurable. And there are features I didn't know I wanted (but are cool) - ability to listen to voicemail over the net, get email notification of voice mail, and ability to take the phone with us wherever we go (that has high-speed internet).
Actually ordering was a bit of a pain - the web page wouldn't work for some reason, so I called. And they have to run you through the ringer, repeatedly telling you about the ramifications of 911 calling. But I sat through it patiently.
However, I'm coming to really dislike the outsourcing of phone services. The lady I ordered Vonage from was a little difficult to understand. And ... she screwed up both my name and the login id. Thus, the title of this post "bigsaceworm" WTF And they can't change it - it's tied to the account number (poor software design there folks). And they had my name as Mcdonald, not MacDonald. I spelled it out for them, and while 'eff' may sort of sound like 'ess', 'em-ay-see' doesn't sound like 'em-see'. It should have tipped me off when she asked if I was the owner of the fast-food chain (and the banter continued with asking me about the king of pop).