When I had a 256k uplink it was REAL bad. I then went to a 1.5/768 line. This sounded line crap on my end (downloads were saturating my Internet connection). I too found that the built in QoS in the DTa was all but worthless.
Not daunted I went out and picked up a Linksys WRT54GS and a subscription to
Sveasoft and I've had QoS up and running in various states for close to a year now.
The 1.5 meg downlink was presenting a problem as you can't really control what is coming down the pipe to you.. only what you ask for (uplink). randomly dropping inbound packets resulted in choppy voice quality on my end.
I changed ISPs again and am now on a 4.0/512 line and it works pretty good. My DTA is has priority over all other traffic in and out of my router and it's almost seamless that I'm using VoIP.
As with anything, you are at the mercy of the infrastucture so adding a half dozen or more router hops between me and Vonage gives plenty of places for things to go wrong... and they do. It is livable for me.
I think the Cable companies that are offering 'digital telephone' service are doing VoIP. It's all intranet to them so QoS and bandwidth isn't a problem.
Doing anything over the Internet is a crapshoot. Like I said before it's VERY livable for me. It is most definately not same/same as what you get from the local telco. The savings for me is consistantly $30 a month or more. I had regular phone bills bettering $100 before I went to Vonage.
The taxes are the real killer with traditional phone service. VoIP being exempt from most of the telco taxes is what keeps 'em cheap.