Alright, first the disclaimer. I have very little basis for comparison to other compressors here... A boss CS3 and a mod'd boss CS3 and that's pretty much it. This could make the review a little awkward, so we'll see how this goes. The other thing is that school's been busy since I got it and I haven't had a chance to go jam with the band at all... so all this is based on headphones, and loud-ish appartment playing.
This is generally pretty straight forward, set the amount of compression you want, adjust the "body" to taste, and have at it. The body control is a little less than intuitive, but not in a bad way necessarily. Generally, it seems like a really smooth kind of compressor. User friendly in that way, but sometimes those wierd quirks that the boss had were fun in their own weird way...
So, I guess the amount of compression is a good place to start. Seems to go from very little compression to pretty squishy... and, surprisingly quiet throughout the range of the control. I was pretty skeptical about this aspect as the reviews pretty much made this thing sound too good to be believed... the kind of review that makes it sound like the pedal will go load up your gear after you're done playing too.

But honestly, when I first plugged it in I had to crank the comp knob to see, and it seemed unnaturally quiet for the amount of compresion and sustain. And that was quiet through my headphones and with my tweeter set flat, so I think it was doing pretty well.
It didn't distort or give any weird artifacts with my fretless' preamp set to a pretty decent level, which was nice. I can't really get as much of the "pumping" or 'breathing" that I could sort of manage with my old CS3, but I think that's more from the lack of separate attack and sustain controls than anything, but I don't think that this was really intended here. At mid-high settings I can still get some dynamics for filters or OD's, but the sustain comes in enough that I can almost sound like I know how to tap... almost. The super high settings crush the signal pretty good, but I'd guess that there's better options for really effect-y compression... but if you hum a few bars this can fake it.
The body knob is where this thing really gets interesting... I'm still not totally sure where exactly flat is on it... Counterclockwise we have some highs rolled off, and a little bass bumped in. It rolls up and seems to more or less flatten out, then just before noon we get some taming of the bottom end, and some clarity in the highs. Not massive loss of lows, but definately less. This increases until in the last little bit of range we get the bass bumped back in along with the high end boost... All this makes the control a little tricky to work with at first, but the effect is subtle enough that it sounds good to me through almost the entire range. No crazy extremes that would be tough to work with, with the possible exception of some of the settings where the lows disappear. I think the body control might have some impact on the attack as well... here's where my inexperience really shows, but I think that it changes somehow and you get a little more purcussive character above noon on the control.
So, in the end it can be pretty transparent, but there's some very useable EQ "pre-programmed" in there if you want it. Does a great job of evening out the strings, I hadn't really noticed volume differences before, but things just feel a little more consistant across the fretboard with this thing on. I really can't give a good description of the body knob, its beyond me to do a good job describing what it does. I guess the example is that I really can't always tell if it's rolled off frequencies or just compessed them differently and brought out different ones.
On my P/J, with just the P pickup when I'm being nitpicky, I can feel some differences in the mids... i'm not 100% sure this isn't just that there's a slight taming of the lows that's bringing out some of the more subtle character of the bass that I didn't really notice before, and there's no way I'd notice in a band. I think I might be being too discriminating here... I've just gotten pretty familiar with that clean tone. With the J pickup mixed in, any doubts I had go away. This just generally sounds great.
My fretless hollowbody sounds better than I thought it could through this... set it up to tame some of the lows (which can get a little boomy) and all the trouble I ever had with EQing that bass is gone.
And... not totally fitting for the site, but on my "tele" it sounds unbelievable. Lots of reviews for this elsewhere, but I can get some really cool touch sensitive compression that's not really noticible playing softly but comes out when I dig in.
In the end, the big test for it will be when I go have a few drinks and play with a band. If it can keep my sloppy playing in check as well as I think it should, it'll be a keeper.
Edited... Hopefully this doesn't sound like too glowing a review, I just haven't really come to any serious flaws yet.