We just had an instructors' workshop, and we were told that the BOSUs lose air quickly, so before each use it's important to make sure that it's inflated. Basically, the more air in the BOSU, the more stable it is. So... if you're going to be standing/jumping on it, you want it to be pretty firmly inflated. You should be able to compress it a little when you press on it, but not too much. If it isn't inflated properly and you stand/jump on it, it can be pretty hazardous to your ankles or knees. I've been using the BOSU in my strength class for the past three weeks, and it is awesome!