My boyfriend and I were checking out undergravel filters the other day , and we decided against it as, it seems more of a hassle to clean rather than having just a filter on the side, as u will have to take it out fully to get under the gravel where all the waste will be, also because we grow our own plants in the tank and would end up uprooting them everytime we took the filter plate under the gravel out - I dont think its very conveniant, but I guess it depends on how big your tank is, and if u wanna grow plants.
I see them somewhat, only in the lfs though. The big names don't carry them. The lfs tried to sell me one but I went with a power filter. When I get my new tank, I'm gonna get a nice eclipse power filter.