Capos: The greatest invention since sliced bread!
The biggest problem I've encountered with capos over the years is when folks (who are playing with others put 'em on too tight, and in the middle of the space between frets. This will ALWAYS make their instruments sharp compared to the non capoed ones. Often the other folks will quickly retune to the capoed one, and before you know it, they'll all be way off pitch.
If you're using a Shubb capo, set it up to fit your neck before putting it on. The instructions (which on this capo are vital), often get chucked out. Don't turn the thumb screw once it's on the guitar. Place it JUST behind the fret, so that it doesn't stretch the string (which it will, if it's too tight and in the middle)
If you REALLY want to get accurate tuning, you can take the rubber off, put 'er in a vice and carefully BEND it to exactly match the shape of your fingerboard. I've done this on every capo I've ever owned....consequently I've got a dozen capos that were bent for guitars I don't own anymore!
Kyser capos (if you can stand the big ugly handle) have great rubber and are pretty reliable, but it strikes me that most experienced players still prefer the Shubb. Me? I favour the Dunlop "C" clamp (quite a cheap one actually) and have cut down a few for use on banjo or Mando-cello.
By the way, folks rarely use capos on mandolin, but if you're just learning, and have mastered the G, C, A, E and D chords, use a capo, and you can play in most jams.
If you're new at this capo stuff, make up little charts for guitar (or banjo, mandolin etc.)
Play G, C, and D on the first fret, and it becomes Ab, Db and Eb. So on and so on up to about the 6th fret. You might want to get an experienced player to make it up for you.
Oh, last thing. DON'T LOSE YOUR SHUBB!! Ah, forget it, everybody does.