UGF's get a bad rap that they do not deserve. Sponge filters and UGF's work by the exact same method, it's just that the gravel serves the same function as the sponge. They both provide mechanical and biological filtration because of the beneficial bacteria that grow either in the sponge or on the gravel itself. I have read that there is a way to make them perform chemical filteration as well (something like putting a carbon cartridge in the riser tube), but I have no experience with that.
One thing that UGF's do, probably better than any other filter kind, is circulate the water throughout the tank uniformly. That keeps the temp much more stable throughout the tank. That's why I really like them. Also, being an older tried-and-true technology, I feel they are very reliable and inexpensive.
In a very large tank, or with a lot of rooted plants, UGF's can be quite awkward. However, in a 2 or 3 gallon tank, it's pretty much a breeze.