I had one, it's a waste of money because while it DOES heat a degree or two it doesn't last long enough to off set the cost of buying it. If you want a pre-set remember that they have the potential to get too hot if there's anything wrong with the internal thermometer and some people have lost fish I believe.
I had the 10 watt Aqueon heater for almost 2 years so I can justify that one. Same with the pre set Tetra one from WalMart. I used that one for about a year and half (I guess it still works, I gave it back to mom where it collects dust). If you only have a 1.5 gallon tank, then opt for a small pre set of at least 10 watts, but preferably a higher.