I picked up the GB-10 last week. I got it the tascam gb-10 to slow down songs so that I could practice. It’s pretty good. You can alter the speed of songs by +/- 50%. It does that without altering the pitch. It does this by grabbing a slice and repeating it. You can hear that pretty easily in the song. it comes across as a wavering effect. On some songs (eg, Tom Sawyer), it makes the drums sound terrible. Other songs (eg, Drives Me Crazy) come out okay. I think that it’s related to how much reverb there is in the mix.
You can also change the pitch. You can shift it up or down almost an entire octave, going up and down a key at a time. Pretty much everything I played with it sounded like blue cheese (you know, just a bit funky) but the pitch was on.