I bought two because I like them, not because one broke.
A youtube review I saw showed that it tracked slightly better than the Boss it ripped off. I haven't done the test myself, but it does track very well. It's possible to confuse the tracking by not playing cleanly, or sliding around. It can get thrown by fret noise and land on wrong or weird sounding notes, but re-triggering notes works fine, and the glitches can be fun if you're into that.
I've compared mine to various EHX stuff like POGs and Pitch Fork, and while the EHX pedals maybe sound more "real," they have an annoying amount of latency, and the Behringer is much faster. The sound is a little weirder, but I find it takes distortion better, and the minimal latency makes the US600 much easier to play.
The biggest drawback is the plastic case. I think it'd be pretty OK with very careful use, but I reboxed one of mine just in case. It was a PITA, but I might rebox the other one some day.
Behringer US600 Ultra Shifter/Harmonist
Can shift +/- 2 octaves or bend +/- 1 octave or other intervals. When harmonizing you can select the key to constrain harmonies. Has an expression