I've ridden one at a riding school in a bungee. To be honest I don't like the idea of artificially forcing an animal's head down. I know it's flexible but there is still pressure. It must be fairly uncomfortable.
A TB mare at my riding school used one! It was because she kept throwing her head up. Now she never throws her head up! I think it depends on the problem
I know two people who use one, it really depends on why. If you are using it to foce the horse to look pretty rather than properly working it into the contact then really you aren't helping yourself as it's likely that the horse still won't be using itself properly as it may not understand the concept of outlines or have the capability. If it's used to aid but not restict (so it's loose and isn't putting the horse 'perfect' but more signalling where the head needs to go excluding the strech) then it would imo be more effective but it will still be able to cause discomfort.
Personally I'm not all for the outline craze, if my horse wants to do it he can but I'm not going to sit there and strap him into a false frame for the sake of it. Overall it's to do with your opinion, horse and use as with all traing aids and gadgets.