I got one of these recently and i like it. Step on it and turn up the bass and you'll [i]shake your foundations[/i], to quote AC/DC. The bass (low) knob controls the output volume of the low pass filter, which has a cut-off frequency of 85Hz. The treble (high) knob is a high pass filter, which has a cut off of 6.5Hz. Yes, they could have made it narrower in size, but it's actually quite light (without a battery, which i don't use). The 'Dry' knob in the middle let's you mix in/out your input signal. I'm recording a couple of tunes this weekend and i intend to use it.
I bought it for my bass, but i tried it on my guitar too (Gibson SG standard) and i wasn't particularly impressed, but perhaps i should have fiddled a bit more. But it really comes alive on bass. When you turn it off and hear your original bass tone, you think hmm that's weedy, lol! I might just keep it on all the time.