I have the Barefoot Arizonia. I love this saddle

. I use it on my heavy cob and all my Fell ponies, no probs. COz they are round ponies

, it slips when mounting, but that is not a prob for me as I have to use the fence anyway coz of an injury. ALso it is the only saddle I can ride in. I have found my cob does not understand my seat aids in a tree western saddle, but responds fab in the treeless. I have also found when my leg stops working (from a accident), he can still get what I want, plus I can ride for hours in this. With a treed saddle, the concussion on my spine and hip was unbeliveably painful, I could just about manage 20 mins (and this was with a very smooth paced horse). It is not an issue in the barefoot. (Not even a fittness issue as I can still only do 20mins in a treed, but then go onto the barefoot and ride for hours).
The only thing against them is I would never let a beginner use one because you need to be a secure rider so as not to cause pressue points. I am also a equine body worker (bit like a physio) and my horses backs are fab with the treeless. but then again they are all native ponies wiht plenty of flesh over the spine. I would not use a treeless on a high withered horse.
AS always, its a case of what works for some, is no good for others, but I highly reccomend the Barefoot Arizona saddle.