First, nicely done. The interface is clean, the progression bar is helpful.
A few suggestions:
It would be nice to have a bell go off both at the beginning and end of the rest period, so you don't miss the beginning of the next 30second sprint. On my phone I only hear a bell at the start of the rest period.
I would also suggest displaying the graphic for the next exercise during the rest period. I work out regularly but I still didnt know what a "push up and twist" was so by the time I figured it out I had already missed 5-8 seconds of the sprint.
Nice to see products like this generating conversation. Keep up the good work.