Is it worth using? I was thinking of getting one to use on my first fermenter for a german weissbier so that the yeast flavour and aroma is more present in the final result. By how many degrees does the belt change the temperature? Are there belts that the temperature can be regulated according to the needs? Do they produce a better result than other forms of heating or do other problems appear?
I have used a brew belt off and on for several years. There is no real way to control it. I suppose if you had a thermowell and a temp controller you could dial it in.
However, it only really heats to so much over ambient temp. It doesn't have a built in theermostat to stop at say 70F. I used to get a good 70F ferment with it when the ambient temp was 55-60F. So you'll need to put it some place cool.
The results are fine, but you have to check it and make sure the place you put it is cool enough to keep it from overheating.
Lastly, the manufacturer of my brew belt warned against using it on glass. The band of heat on the glass can weaken the glass as the cooler glass around it fails to heat up as fast.
Brew belts are cheap and easy tools. You just have to experiment with them a little bit.