I ran one of those filters on a 5 gal. (the Aqueon bowfront LED kit) with excellent success. I actually had to turn it down quite a bit as it was turning the tank into a whirlpool! For a short time the tank was housing two guppies, a swordtail, two tiger barbs and two corys but the system had no issues at all and I kept up on the water changes.
I recently tore down that tank and moved the barbs and sword into my 20hex and the corys into a newly set up (and my first planted) 20H tank. I ran that little filter on the 20H (set wide open) for a week or so with the two guppies and it did fine. It moves a pretty good amount of water. Now the 20H has the Fluval C3 (and the rest of my corys) from a 10 gal that I also took down. The 5 and 10 gal. will stick around for "backup" or QT use now.
The only complaint I have about the filter is the cost of the cartridges ($10/pair retail). They are quite small, but overall the filter has a decent amount of surface area for bio-filtration considering it's size. The filter is also silent, so it is perfect for a bedroom tank.
Chris
