I bought my device several months ago at the Apple Store when it was first released. I bought it mainly on a lark thinking it would be fun to play with. Well I was right! It is fun to play with. Active Body developed some games that use the Activ5 as a controller... pretty clever, and fun.
First and foremost in my mind is the portability. This small palm-sized device can be carried anywhere. I don't have to wander down to the fitness center at the hotel anymore. I can work out in my room in my skivvies! The workouts are predefined and well illustrated in the software. The software presents the exercises (challenges) in themes tracks and lengths. For example "Watching TV" or "At the office" or can target specific regions like "core" and "upper body".
This thing turned out to be so effective for me that I carry it to work and when I travel. It's pretty effective at relieving stresses of the day with just a little exercise, or game. So what I bought on a lark turned out to be pretty indispensable.
Is it perfect? No. One issue I have with it is I sometimes have trouble figuring out how to hold the unit. This was mostly when it was new, and experience has helped my find "my style". The illustrations in the software are good for positioning but sometimes vague as to orientation. (Again experience will help with this.)
Another concern I have is the battery cover. It is designed such that you snap off the whole one side (pad) of the device to get to the AA battery slot. I worry about longevity of the plastic snaps (as with other products) but considering one AA battery lasts up to a year I don't imagine this will get a lot of use. All in all, for the price and convenience, I have noticed improvement in my posture and general strength. Being so small I carry it nearly everywhere and that alone encourages me to exercise (play) more than I normally would.