I want more info on this too. I was thinking about picking one up to replace my Tascam CD trainer, but I've read a few reviews of the guitar version of the mp3 trainer which give it low marks due to freezing / software instability issues.
Since I posted I looked around and apparently it's a tad flaky. It only wants your mp3s in relatively low bitrates so you need to fiddle with it and possible reencode everything so it actually sounds worse in order for it to work. Once things get into the device itself apparently the sound quality leaves something to be desired. Just like you I've only been able to find reviews of the guitar model which has also apparently been out longer.
In the end I'd suggest trying to find a place where you can return it if it doesn't work out. I'm pretty sure that Musician's Friend will take it back as part of their "45-Day 100% Satisfaction Guarantee" minus shipping.