i use on guitar and synths for compression or limiting
the pedal is really good, based on an ibanez unit ive never used.
it works as intended in a simple way with a good sound quality
and it is versatile, it does hard limiting "max sustain" through to milder compression
attack control is fine, its senstive enough and has enough range for general use.
its very good value,
if youre not sure why you want this pedal, it will crush (compress/lmit) the transient of a sound at a selectable "ratio" - using the sustain dial, (amount of crushing the initial peaks - flattening the soundwaves level, making it sound more "sustained") with a dial for how quicly the crushing will take effect when you play a note (attack rate)
it sounds good after a distortion or OD pedal, or on a synth saw wave for example, and ive used it on drum machines.
it will be useful to anyone who needs a comp/limiter. regardless of its low price.
behringer also produce the cs400 (boss comp clone), which is also very nice, id recommend both, although the cl9 is more agressive, and basic maybe. very good effects unit.