I think it would work for a goldfish or guppies, probably not a betta fish though....bettas need a heater which is hard to find for a tank smaller than 5 gallons, plus they need surface area at the top. It depends on what type of fish you are getting, but it seems like a good deal.
I had such a hard time finding a heater for the 2.5 gallon that I had! I ended up getting a five gallon tank. If you have a heater for it I think it would be a good deal....seems like it would be the same as if you bought all the components seperately, just cheaper since it's a package deal.
I'm sure somebody has tried it, or something similar. If I'd known about it I probably would have gotten it.
Erin I use the tetra 2-15 heaters from wal-mart they are $14 and work great. I have 5 of them. One in each betta bowl and one in my 8 gallon biOrb. All tanks maintain a temp of 78 give or take a degree with the room temp.