I use a capo for a lot of stuff, but it's mainly to move the voicings I like best for the song to the key that best fits my voice. Sometimes if I'm playing with other people, I'll capo up really high, but if there's a mandolin or anything similar, I don't bother, because I'm not going to invade the poor mandolinist's sonic territory with my much louder instrument.

I'll go up to the fifth or sixth fret, but beyond that, I consider using different voicings unless I'm going for that really high "music box" sound.

James Blunt personally offends me with his capo use, btw.