Here is an article basically advising against them:
I think UGFs are one of those subjects, kind of like the product "Cycle", where some people like them and others hate them. I had a tank with a UGF many years ago. However, at that time I had no idea what I was doing and so when I did water changes I drained the entire tank, removed the gravel, scrubed the UGF plate, etc. Consequently, I never had problems with the filter become clogged as they discuss in the article above. However, totally draining the tank and killing off all my bacteria on a regular basis was clearly not the smartest fishkeeping move in history either. :-\
I think an UGF can work well if you know what you are doing. Obviously it needs to be cleaned like any other filter and this is where most people base their judgment on the UGF. They do not clean it... It stops working.. So it is obviously the filters fault right ?? LOL!!!
So if you are going to clean it.. Usually once every 3 to 4 months of use you'll be good to go. If this is to much hassle for you to do then I would stick with a nice HOB filter which work very well and are easier to clean. ;D
Just my 2 cents..