I'm going to have to say yes, they are a little weak. I would not use one on a commercial sized grow, lol... they make hot water heater sized filters for that!
However they are re-useable and keeping them clean according to the instructions should make them work at their best. I think for a small PU grow, like a closet CFL or moderately sized HID grow, they are a great product. I also set onr up as a scrubber, jsut stuck on a spare inline and hung from the ceiling. It's nice how little they cost, that you can keep an extra one kicking around for whatever.