I have a Hydor mini heater and really like it. It's a fixed temp heater, so it's only to be used in a small tank when the room temp is under about 75 degrees. I have a 2.5 gal tank for a single male betta (Claude Monet) and it keeps his tank at a comfy 78 at night, then with the light on during the day it goes up to about 80. I have it under the gravel, under the filter - one evening I saw Claude hovering between the heater and the filter intake (maybe he thought it was his own hot tub! <img src="/images/graemlins/smile.gif" alt="" />
I'm in FL, so we only use it periodically but it's also great for heating up a changing bowl when cleaning the tank, then heating the tank water to assure a good match on temps before putting Claude back in the tank. I check the temp in the tank multiple times a day as well.
Keeping a small tank toasty is a challenge, but worth it to keep our babies happy!
Cheryl