Has anyone lost weight using the ab circle pro?

It's not a weight loss device. There is no way you can possibly use enough calories with it to lose weight. At the best, you may strengthen some of your core muscles, which will help you look a little thinner.

I've heard a few people say it's fairly difficult to use, your knees tend to slide off the pads if you go fast on it.

To lose weight, you have to get your diet in order, and do regular consistent exercise.