I use the belt in my basement to warm my brew up 4-5 degrees as it normally sits at 63-64 degrees which may be cool for some ale yeasts.
How do I only do 5 degrees with the belt you may ask (as others have overheated their brew from what I've read)? Well, I've attached it to one of those household light timers. I cycle it on an hour, off an hour (sometimes off 2-3 hours during the daytime), and it seems to not overheat my brew...
Seems like a good solution to me as the belt will otherwise give you X+10 degrees or whatever. This thing really needs some kind of thermostat to be awesome, but it will work for $17.