Adapter: Linksys RT31P2
ISP: Qwest DSL (1.5Mb Down and 512Kb Up)
Setup: Behind a home-grown Endian Firewall router. Adapter is behind the router, on a separate "orange" subnet, with holes punched in the firewall for the recommended port ranges.

I have not noticed any problems with Qwest, although I have only been with them for a week. I had another local provider that was terrible, with pings in the 150ms - 300ms range, and the adapter had problems staying provisioned. I literally could not use the phone at all. It was just completely unreliable.

With Qwest I haven't had any noticeable problems, although I'm not home during the day, so I haven't "monitored" it, but my wife would definitely let me know if there were problems with the phone.