Bought one of these for my 5gal, and it works great. I think some of them work and some of them don't. For my 5gal it always keep it at around 78F
But in my hospital tank, it keeps it at 76F which isn't too bad, but I'd like it to be 78F. But still works pretty nicely. However, didn't really trust it for my main tank, so I got a heater that automatically keeps my temp around 80F and i love it!!
It's around $20 but worth the buy, and got it at petsmart. I can't remember offhand, but i'll let you know when i check it.