When playing a new song, and it is in a key that doesnt work for the singer / singers i use it to move the key around easily. Try to find an open key and then play the song without it. Sometimes A won't work and Bb is the only key that works for all the singers, and playing it with a capo just sounds better.
I'm waiting on my Shubb for my lap steel, I won't use it often but it's easier and quicker than tuning from open G to open A for 1 song and then back. I have a Kyser for guitars but I might get a Shubb for that also.I use Shubb capos on my acoustics.
Bill
They're also very useful for doing electronics work on a Strat. I put one on the first fret, then just pop the vibrato springs off, rather than loosening the tuners. It keeps the strings wound around the posts, so when you put the springs back on, the guitar stays in tune better.
I used to use them to impress the chicks, but it kept changing my tuning, so I returned it.