I had one once... it's still kicking around at the bottom of a supply box somewhere.... Unless they've dramatically improved it, or there's a different design, I wouldn't use it again...
There is a certain line where the water level has to be for it to operate right or there is no suction. BELOW that line is the out spout where the little bag for dirt is (i.e. the dirt bag is in the water). The bag is like a media bag or aquarium net. Granted, there is a lot of force by the vacuum, and as you vacuum, the bag really puffs up, but it is blowing stuff (and water which must pass through so you can't have, say, a solid/plastic bag) and VERY small dirt then passes through as well. Within a short time, the whole aquarium is disgustingly cloudy! It does settle and clear, but I couldn't stand it, I felt so sorry for my fishes.
edit.... thought I should also mention, that I had tried to improve this by putting a wad of filter floss in the vacuum bag to catch the finest stuff, but that didn't work.