I have this Marineland Stealth 10W. Be advised that the 10W is NOT adjustable like the other Stealth models. However, it fits perfectly into my 4 gallon Biorb (despite the curve of the tank), and it has been keeping the water at a constant 78 degrees. I'm very happy with this heater. I bought it at PetSmart. Can't remember the exact price, but I'm pretty sure it was under $20.
Before buying the Marineland heater, I was using the Hydor 7.5W mini heater pad. The room the aquarium is in is very drafty, and the Hydor mini couldn't heat the water above 70-72 degrees (it will only increase the water temp. 3-5 degrees). I like that the Marineland heater has a thermostat that will shut off the heater when the water reaches the proper temperature.