Fantastic concept.
Two things I'd like to see that would make this a 5-star app:
1) better, more clear controls for linked vs. Independent tempo clocks; scrolling around with my finger seemed to randomly toggle between the two. Would prefer a direct input field where I could type or select either the numerical value or note division.
2) The ability to use this as a master MIDI clock out with each tempo as a separate channel... Oh my gosh, just the thought of that makes me drool. That alone might honestly take it to 5 stars for me.