I think it's definitely worth getting the DVD, if only for the premixes. There are a bunch of different circuit premixes (as well as cardio only premixes), but a bunch of them do involve Step Blast.
Incidentally, the foot work in Step Blast is a bit tricky at first, but it's so worth learning. Step Blast is one of Cathe's all time greatest step workouts (in my opinion). Just tackle it one section at a time and you'll get it in no time.
Shelbygirl