The RAT type of distortion is unique and this pedal does nothing but deliver it purely.
As far as the construction goes, it is bulky and, despite sturdy, could be finished better: the led isn't centered, the edges are roughly cut and there is all kinds of rattling when adjusting the knobs, so any tweak in the fly is out of the question.
The RAT sound is what it is, if you like it I find it hard to find a better pedal than this one. In the lower settings you'll be able to get a nice treble booster kind of sound, very raw but also great to find lost overtones allover the fretboard.
Anything farther than 10% on the gain knob will enter definitly into distortion kingdom.