I've heard nothing but great things about
Tascam's CD-GT1.
It's one of the newer guitar trainers on the market and it works well for many reasons. You can speed up and slow down any given track without changing pitch. You can loop certain sections of a track over and over again. It's got a chromatic tuner. The only sore spot I've heard of is the fact that the built-in effects aren't exactly world class, but they're just bonus anyway.
I'd go for it. Also available (for bass players) is the
CD-BT1 in a much cooler black color. They're available through most stores and you can probably snag one on eBay for about 150 or maybe less.
Good luck.