Was just toying with the idea of getting one as although the stock fans are fine for keeping it cool, when gaming temps can climb (nothing to severe) but seeing as these can be bought cheaply I was tempted.

Any advice/comments on them?

I have no idea which makes/models are any good, and if they are worth getting at all.

I have a Lapcool2 which I got when I had the original XPS. With the 1710 I just leave it off. Its nice though cause it overhangs the cooler so the fans stay a couple of inches off the desk. It also angles the laptop downward for easier typing.

As long as your fans are kept clean, with compressed air sprayed at the back vents, or opening it up and cleaning, then I wouldn't worry.

Someone suggested just getting a couple of bottle caps to raise the laptop off the surface its on. Or you can actually buy stick on legs for laptops that prop it up.

Water bottle caps x 2 will do the trick.

I never cared for USB powered coolers I have owned all off Dells Highend laptops 9100, XPS , XPS Gen 2 and currently the XPS M1710 and have used laptop legs on every one and beleive me they do work it lifts the rear of my laptop to improve airflow and also puts my laptop on a good angle for typeing.
http://www.laptopdesk.net/access-legsf.php

I have a D-Dock I now use with my M 1710 but still have the laptop Legs I had to put the Laptop Legs in a special spot and drill a hole for the wireless card screw and trim a little with a dremel so the Laptop legs would not interfear when docking but worth the 10 Minutes of drilling a whole and triming the Laptop Legs as they are a great product and do work. I also use 3 M weatherstriping glue rather then the 2 sided sticky tape that comes on the Laptop Legs to make them permenantly attatched to my laptop as with the 2 sided tape will weakin after 6 months to a year or so.