Phone service quality is really a subjective thing, and hard to put a number on exactly. Unfortunately, to me the voice quality of the system was just so-so, even with the Vonage "bandwidth saver" feature set to highest sound quality (which is the default). I could always hear background noise that sounded sort of like a modem carrier signal or a fax machine trying to connect.
After doing some investigating, I found that others had the same problem. Curiously, if you connect a phone to the second phone port and take it off the hook, it clears up the background noise of the phone on the first port. By the way, the second phone port is only there in case you have two Vonage phone lines. So if you only have one Vonage line, then you won't be needing the second port anyway.
I also found a slight echo on the receiving end of calls sometimes, and a very slight delay when talking, just like with some cell phones. The end result of all this, at least for me, is that it can make talking a bit tedious, but then again, talking is tedious for me anyway.
There are a couple more quibbles I have that I should get out of the way. The "More.." link to online help on the QoS page is dead, which isn't very helpful. And there doesn't seem to be a link to the WRT54GP2 on Linksys' website, and searching for it with the site's search function turns up no results (though a similar model, the WRT54GP2A-AT is there). Of course, Google found it here.
The Linksys WRT54GP2 retails for around $150, but you can currently find it on line for a little over $100. This is about twice what you'd pay for a plain old WRT54G, but remember you're also paying for the built-in two-port Analog Telephone Adapter (ATA). But the GP2 is just the razor, so to speak, and the blades - the Vonage phone service - will run you anywhere from $14.99 to $49.99 a month, depending on the service plan you choose.
The bottom line for me is that the GP2 is a very good, feature-rich, hardware based firewall / router, with decent wireless performance and QoS features that are essential for setting the balance between VoIP and other bandwidth-hungry applications. But I'll be keeping my old telephone.