I love this app!
I am a professional musician who has a hard time staying "in the pocket," so it is important for me to practice with a metronome. Tempo lets me maintain my rehearsal and set lists with the tempo for each song, send them to our drummer, and send them as text files to the computer so I can print them out for our luddite technology-averse guitar/dobro/mandolin player.
On stage the drummer keeps the set list on his iPhone, so he can just tap the song while Im setting it up with the audience, then he can use the blink (click muted) to grab the perfect tempo and count us in.
The recent update and redesign provides an ideal interface for the iPad, so editing set lists is a pleasure. There are different themes you can use depending on stage conditions and what you like.
This is hands-down one of the best investments any musician can make, and possibly the most valuable app I own.