Sure, step is good for the heart. More importantly, though, it's good for the butt. An ACE-commissioned study shows that "step-ups" are as good for the gluteal muscles (less scientifically known as "the booty") as traditional squats, which many have difficulty performing.
Although the workout we get in a step class is without hand weights and far from simple stepping up and down, there's no doubt that while you're sweating that ass off, you're also making it look much nicer.
Thanks to Barbara for telling me about this article. (Barbara takes butt fitness very seriously.)