Hey DB!! I use a Kyser; but, I wouldn't recommend it. It's too hard to get on and off the neck. I got it because I use a Kyser on my guitar, with great results. In general, I don't see a problem in using a capo on the mandolin, especially when you're playing in a group. I'm with Willie-O when he says to use the closed position chords.

All the best, Áine