I've got a Tascam MP-BT1, and it's does exactly what it says on the tin. A few comments...
The speeding up /slowing down is crude, and I find it hard to pick out the bass line when it's slowed down.
The changing pitch works well for a semitone or two either way, but beyond that , the don't expect miracles !
The loop feature, which lets you cut a short section out and repeat it over and over, is really useful. With practice, you can get it to cut really accurately. I use this a lot.
The bass input , headphone out and mixer mean that you can practice without an amp. Sound quality is good. I use it like this most of the time, with decent headphones.
The bass boost is not perfect, but you can normally get something if you fiddle with the settings.
The bass cut is very crude, like listening through a cheap tranny.
Has lots of effects built in - Don't see the point, and have never used them.
Also has a tuner and a metronome built in. Both work well.
The user interface is very 1990's and clunky, but works OK.
This may seem silly, but having a easily accesible FFWD/REW buttons make life much easier than a standard Ipod / MP3 player.
Seems solid and well made, and the recharagble battery is replaceable if you really wear it out.
Utterly indispensable when learning a long covers set, like I did recently.