I tried many compressors in the studio and found out there are a few things that I didn't like with most of them.
*real low end is rolled off
According to Ovnilab.com, many compressor pedals are Ross clones, which supposedly suffer from this problem. That said, my guitarist is a pro sound engineer, and he loves the Keeley (a Ross clone) on bass. His top pick for bass is the Joemeek (not a Ross clone, AFAIK). But I'm completely satisfied with my Markbass Compressore, which he also likes.
Here are some reasons to choose the Compressore:
-Full featured (DON'T be afraid of the knobs, LEARN to use them).
-No loss of bottom; in fact, has a very warm and full bottom end (I like my comps like I like my women

)
-Has an indicator light that shows when the comp is kicking in, which many of you have stated that you like about other comps.
Here are some cons:
-The thing is friggin HUGE. Not pedalboard friendly. I do have it on my pedalboard, but recently balked at a VERY good deal on the Markbass Synth pedal, because there was no way I was going to have two of these monstrosities on my board.
-The power requirements all but force you to use the Markbass power supply, which is not particularly well-made. The cable is quite flimsy, and I could see needing to replace it from time to time. And if MB ever goes out of business, you'll have a hard time doing that.