Go to a store and try one.
Read a very accurate review here
This one of those pedals that if you buy a used one you can always sell it for what you paid.
What do you want a compressor to do? The Diamond is a great always on tone enhancement tool if you're looking for that very subtle evening out of your frequencies that makes the bass pop in a mix...it's just wonderful for that. If you need a really squashed obvious effect once in a while say with distortion or an octaver or want limiting this isn't the one.