There is tube overdrive emulation on the input gain stage, never intended to be a massive break-up or distortion effect but more for guys who are looking for some of the natural harmonics and compression that are inherent (or can be) when overdriving a typical tube front end.
For distortion, I would recommend finding one of the thousands of pedals that suits the player's preferences.
I love it. With my normal playing the sound is so thick and authoritative, then when I dig in it has a nice crunch. Of course my Roscoe basses help that a lot, but they don't crunch as nicely on other amps as they do my D800!