I have one for my mare, she goes really well in it. The one thing I don't like is that I feel like if I'm riding with significantly contact for any length of time (like, we're on the trail and she's trying to bolt or something) it doesn't release as much as I would like. It doesn't seem to bother Lacey at all though, so it might be all in my head. Especially since she's one of "those" mares that will let you know, very swiftly, when she's displeased. Haha
I've been using mine for about 9 months and I'm really pleased.
I bought mine from this guy: Pony/Cobb Bitless Bridle Indian Hackamore Color Noseban - eBay (item 260591860932 end time May-24-10 21:44:07 PDT)
And I haven't had any sort of slippage issues, of any sort.
I've even galloped in it without any issues. My mare was taught to be a little less than respectful in a bit (nothing terrible, just not as light as I would like) but she hasn't had any of the issues turning/stopping in this that she has with a bit. Now, she was fine about turning/stopping in a bit, there was just something lacking that I gained with this shindig.
However, these are definitely something I would only advise for someone with really soft giving hands. You can do a lot of damage to your horse with any bitless contraption so it's not something to just switch to. Especially since these indian bosal things don't really release a lot, you just cannot be always pulling on your horse. That's my only caveat. :)
I actually just switched my mare to a sidepull type deal, for the summer, and she's doing even better. However, she does mostly everything just off my leg nowadays so I'm not really sure how she would have done "before." Currently, I'm liking this better because I know that it can release for sure. And, since I put the rein rings on lower, it doesn't shift around all over the place like the kind with rings at the nose knots.
Here's a picture of my sidepull thing I made (just to give you an idea of other things you could try):
But in any case, good luck!