The instructors and staff. They are all really helpful and understanding of individuals needs. Sometimes people use an incorrect technique but the instructors go out of their way to show folks how to do it correctly. This helps keep injuries at bay and maximizes the effort one puts into the classes. The music is motivating and helps keep me pushing hard. I enjoy the classes so much that I cancelled my regular gym membership.
As a runner I really enjoy the strength and flexibility training aspect of HIIT.
The one week free trial is good for someone who is in decent shape. Otherwise consider trying for a month to let the body acclimate to the variety of fun yet challenging routines. You won't regret it and you'll feel good about yourself both mentally and physically.