Alesis 3630???

I would say it is not a "good" compressor, but it does what i does well. It is more of an effects compressor in that you can really tell things are being compressed. That works out well if you are planning on using it for things like sidechain compression, where you want to hear a lot of pumping. 
If you are looking for a good sounding compressor that is a little more of an all around unit, I would recommend the FMR RNC. These are much more useful as an all around utility compressor.

I just noticed you were looking for a gate also, I would just look for a used DBX in that case.

i'm not looking for a vocal compressor.  i wanted to use the sidechain on one side and use the other to cut some fuzz from a tm7 distortion pedal.  i've played with a dbx 166xl ad was impressed but didn't want to spend $200.  the 3630 seemed like a cheap alternative.

the RNC is fantastic for general compression use. the side chain works nicely on it too. has anyone used the RNP8380 before?

I don't know about the 166XL, but I think the DBX 166A is what you want.  The XL does not have a release function on the gate.  On the 166A you can chop the sound 100% when the gate threshold is up all the way, and the gate release is down all the way.  Then put a sound(16th note clicks or whatever) through sidechain and you get a very clean chopping sound.  I've tried this on a berhinger, and the RNC compressor and neither will produce the same effect, the chopping is not 100%.  The noise gates on logic pro do a great job of this, but on logic express you can't use sidechains.  I'd like to try this on the Alesis 3630 myself, seeing that they are so affordable.  Maybe it's a noise gate that your after, and not a compressor.