The Boss TU-80 is a pretty cunning piece of kit. In its tuner mode you can select between "Chromatic" (tuning by any of the 12 steps in the chromatic scale), "Guitar" (tune by string name) and "Bass" (also tune by string name). It'll happily cope with 7 string guitars, 6 string basses and flat tunings, too. In metronome mode you can select rhythm style, beat and tempo - I've found this functionality to be *really* useful, I have to say.
Input is via standard quarter-inch jack, or you can use the built in mic for acoustic instruments. There is an additional quarter-inch jack for output, so you can use the device inline, where it will operate in true bypass mode. All jolly good.
Problems? Well, yes, a couple. The metronome isn't particularly loud, and you can't output it via the output jack. I appreciate the occasions you'd be likely to want to shove the metronome through your amp live will be next to nil, but Boss might have considered letting the output double as a headphone jack, perhaps, so you could hear the thing over anything other than an acoustic. On the subject of acoustics, the internal mic isn't insanely sensitive, so may be a pain in noisy conditions.
Overall? It's a very handy piece of kit. Personally I'd view it as a home practice tool that you can also get to double as a stage tuner in a push but I don't doubt that other people will be more than happy to keep one in their gigbag. It's not exactly going to take up a lot of space! All-in-all, a good-value, well-made product from a respected manufacturer. You can't go far wrong with that.