I did the stability ball section yesterday and thought the exercises were fun and effective. I could really feel them, but didn't think the workout was too hard. There is absolutely no rush to this workout and she gives you a bit of recovery time between exercises, which allowed me to regroup and really concentrate on form. There are 3 premixes: Balanced Core (22 min), Complete Core (21 min) and Tough Core (18 min), so I think there's probably something for most levels.
For a 20 min workout, the time flew!