Oh. My. God! Does that sound good!
I tried premastering an unfinished mix yesterday, just to see how it would go, and it instantly got growly and ballsy as frak. I can't comment on compression much, because it was an ambient song without any percussion, but it made everything warmer and clearer at the same time. I still need to try it on more percussive music, and try out other tube models (because you can totally do that, apparently).
The build quality is literally palpable, when fiddling with the controls.
My only complaint is that I found impossible to match dry and wet levels, rendering the 'bypass' switch about useless for before/after comparison. Maybe that's just me, and I need to learn to use it better, but anyway, that's the only bad I have to say about it.
EDIT: After a couple of years, I'd like to add a couple of comments: 1. You can totally make it super slappy of percussive tracks, as well as the opposite extreme- that is, not noticeably pumping at all. I still can't explain how to get one effect or the other, except by fiddling with the controls until you're happy. / 2. Since I tend to run it hotter than it's meant to (I like when tubes cry for mercy), I still can't get consistent gain staging, which renders the bypass switch useless. / 3. While swapping tube models is super easy, adjusting the meter's calibration to a different set of tubes is not. Thus, instead of fitting tubes to the kind of music I'm mastering, I just leave my favorite set of tubes in there, and not play with swapping them anymore, which is a bit of a pity. But I still love this unit and wouldn't trade it for the world.