I can not stand my Rena Cal Top heater, the thing varies like +-4 degrees and it's set to 72 and the tank hovers around 78-82!!!!???!!! 100w in a 20g so it's not to small/big.
On the other hand my Stealth equipped tanks are much more stable and nearly the temp the gauge on the heater shows.
I also have a Hydor 300w inline heater and it is great too.