I've always been an ATC user, and until recently always used an ATC-XP. A year or so ago, I started using a Petzl Pirana, which I really love because it is so easy to rig for different amounts of friction, plus never comes off the harness when disconnecting. I also like that you can adjust the friction on the fly - having lower friction at the top of a long rappel, and greater friction at the bottom.
The Sterling Rope ATS device is much newer, and has a lot of similarities to the Pirana. It's regularly about $40 online ($5 more than the Pirana) but is currently on sale on Amazon.com for about $23. I've ordered one, and it should arrive within a couple of days. I'll report back after I've tried it out.
What is your favourite rappel device?