I'm sure there are even better Pulteq clones out there, but as far as I'm concerned, this one is already good enough for mastering.
Since it has no tube, the low end is a little bit weaker than in the 19'' rack version, but still excellent. Now, having two different frequency settings in the low end is a very, very convenient feature.
The high end is where it really shines: It's super clear and smooth as silk.
The pots feel sturdy, and their being stepped is excellent for setting recall.
I totally recommend this device. I couldn't do without it, now that I've tried it.