I have two. they're okay at best. one of them periodically stops turning. they might work well if you put the full 300gph through them, but otherwise i don't notice much of an improvement to flow in a 30g tank. but for the price, you can't really beat them. i'm sure it'd work great for a 5-10 gallon.
for larger tanks, check out this as an affordable circulation option. also made by hydor.
http://www.drsfostersmith.com/Produc...004+22788+2035
and the new tunze nanostreams look awesome and they're cheap. i should have mine in a few days.
http://www.jlaquatics.com/phpstore/s..._ID=tz-tsn6025