In my most recent experience, I would have to say any DSL ISP-supplied equipment (routers and modems) are generally inferior to other ones. For the past few months, I have been troubleshooting a connectivity problem for a friend, and have done everything to address the issue, only to be told that she has a bad modem (she has a Siemens Speedstream 4100 DSL modem that was supplied with her AT&T DSL service when she switched to it two or three years ago). Last weekend, I did some online research to locate a new modem for her and found numerous references to frequent issues with Motorola DSL modems that ran hot, 2Wire Home Networking Portals that dropped connections repeatedly, and Speedstream 4100 modem that also dropped connections and needed to be rebooted at least once a day. All of these devices were used as standard-issue equipment by AT&T DSL and were known to fail anywhere from a year to every two to three years and need replacement (at the customer's expense, of course). Also, I found that any DSL modem or router that is supplied by and made specifically for a DSL service provider will most likely have firmware that is not upgradeable as the firmware updates would most likely have to be either written by or approved by the ISP. I know this to be true because I used to do tech support for what is now known as AT&T Yahoo! DSL back when it was just SBC DSL and there were never any firmware updates that were available (nor even suggested) for the equipment they supported.
Third-party equipment that is not supported by the ISP is generally better, but you lose the "advantage" of having it supported by the ISP as having such equipment (and letting them know you have such equipment) gives them the easy way out by telling you that you have to contact the manufacturer as they do not officially support that equipment. For those of us that have a somewhat better understanding of such equipment, that does not pose an unsurmountable problem. I am going to be installing an ActionTec GT1701D DSL modem (not ISP-specific) on my friend's DSL line this afternoon and (if the reviews are correct), she should have a stable (and possibly faster) connection for the next four to six years. I am interested in seeing the results firsthand.
As for cable ISPs, I have yet to have a bad experience with Cox Communications' supported equipment as my Terayon TJ-715 cable modem worked fine for about 4.5 years before dying and I had it replaced by an Arris telephony cable modem when I upgraded to their digital cable service and added their digital telephone service on to my service plan. I have had the Arris telephony modem for nearly five years now and have yet to have an issue with it. Note: I did pay for the TJ-715 cable modem, but the Arris telephony modem is most likely being rented by me as part of the service as I don't recall having to pay for the telephony modem when I had it installed.