I have Cox residential service, and my fiancee uses Qwest DSL at her business. I have never had any real issue from my Cox connection, but her DSL is terrible in various ways.
* Modem always starts out at full provisioned rate and throughout the day gets slower and slower (according to the web GUI)...perhaps noisy line?
* Modem constantly reboots itself during the day, which also cycles the built-in 5 port switch. This was having the wonderful effect of disconnecting her POS system from the main fileserver, as well as disconnecting and disrupting ATM and credit card transactions. I "fixed" this by installing my own 8 port switch, and moving all the network connections to my switch, and uplinking the DSL modem. At least now when it decides to reboot itself, it doesn't DOS the rest of the LAN.
* Any device (phone, fax machine, alarm system) that picks up the phone line causes the modem to go offline and retrain, even though using the supplied filters on all of said devices.
* Wireless is flaky and constantly drops (probably from the modem rebooting itself)
* External IP constantly changes every 15m or so. I suppose this could be happening when the modem reboots itself and it DHCPs a new address from Qwest. DDNS has been a lifesaver here.
It sounds like all of this could be fixed with a new modem and maybe some analysis of the line. It's not my account so I can only do so much, but I know she has complained to Qwest and so far nothing has been done. You would think a business account would get better service.