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.