Rheostat...

I got a Zoo-Med rheostat to control the temperature of my UTH. Has anyone used these or know a whole lot about if they work well?

Save up and get a good thermostat such as Johnson or Ranco ASAP.

Rheostats are fine for the 'meantime', or if you have your enclosures in a room where the temps are really stable. Otherwise, you really want a t-stat. ZooMed rheostats vary incredibly from unit to unit, as well, so it's hard to tell you just how reliable they are. I have about 5 of them as backups and such, and each one holds the desired temp at a different dial location.

Just make sure you check on the temps as often as you can, and adjust the dial as needed. I found that when I was waiting for my t-stat and had to use a rheostat, the best thing to do was record how long it took the thing to reach the desired temp and then drop 2-4 degrees then go back up. I used those calculations to set a timer and put it on that. Worked fairly well for about a week or two.

I think the ZooMed rheostats are complete pieces of junk. IMO, the $10 lamp dimmers sold at Lowes work much better at half the cost.