I haven't used one of these amps at a gig, but I did play through a couple of them at Guitar Center for about an hour, on about three different occasions, trying every electric acoustic guitar in the store.
The Loudbox has a nice, natural sound, good tone control, and nice-sounding, but subtle chorus and reverb. If you're playing an acoustic/electric it will definitely sound good for you, and they're light and portable.
I didn't try a mic through it, but I think a good mic would probably work well with it at moderate to low volumes.
Fishman makes good-sounding guitar pickups and electronics. The Martin I ended up buying has a Fishman active pickup in it. Sounds great!