as a lead guitarist that plays one off gigs here and there with singer/songwriter types (who play the same songs in different keys to accommodate how their voice feels on that particular gig) i try to avoid them pretty much always. i get thrown off when it comes time for leads and i'm playing chords like i'm in A but it's actually in C or whatever and i start soloing in the wrong key. i'm more comfortable just using barre chords, i may miss out on signature licks that require open strings, but at least there's no confusion for me as to what key i'm in. anyhow, i think aqualung uses a capo also.