I recently posted a thread in the "Tack" section of this forum, and got a few responses. I've been looking into buying my own, and have been using it on my horse for about a month or two now.
It helps them build the correct muscles to help them hold a frame, and encourages them to use their hind-end more efficiantly. My horse was rather 'rushy' and carried his head way up, hollowed his back, and was very heavy on the forehand. However, I've been working him in the pessoa [we call it "pessoaing" at my barn] 1-2x per week, just depends, and he's already traveling straighter, using himself more effectivly, and is building lots of muscle.
Be careful with this system though, it can create havoc if used incorrectly. If your working with a horse that is very spooky on the lunge, don't use it on him. I like to let my horse run around or lunge him very lightly at first to get him used to things, because if they spook with the pessoa on, and are not used to it, they will flip-out and most likely injure themselves because they feel confined.
Don't every overtighten the 'side-reins' on it. [Not sure what they're called exactly] I like to leave them loose at first, and then tighten them slowly as the horse gets warmed up. Only used for 5-10 minutes the first few sessions, and then gradually build up, but don't pessoa for excessive amounts of time, its very hard on the horses legs to lunge for that long. Don't move to fast either, because it can cause alot of damage within a horse.
But overall, I LOVE THIS SYSTEM! And would highly recomend it to anyone, but only if they're very experienced. I still asked my trainer to check over the entire system before I start, because he's more experienced than I am. It can be very dangerous in the wrong hands, but in the right ones, it is a veryyy effective tool!
Hope that helps =]