I've used it and personally didn't think it made a difference. Have got the big one on Magnitude now, which is a quarter of the price and lasts just as long and I actually find it better for mine. Having said that they all work differently on different horses anyway...
It's worth experiementing, I use cool calm and collected on Fudge (it was recommended to me as he is a bit irrational and struggles to concentrate) in most situations I find it has helped reduce the irrational reactions and I think it has improved his concentration, however it hasn't made him a dope on a rope but according to the blurb it's not meant to do that. Several people I know use Magic and find this works well for them, so I think it's pretty much trial and error until you find what suits your horse.
I used it on Axel and it made a massive difference to him, we could actually ride through a whole dressage test!
However I dropped nearly a whole tub on the floor, and was so upset at wasting nearly £30 I couldn't bring myself to buy another one. However Axel has not really been any different day to day, but was much more stressy at a competition.