Celeste I was glad to see this thread, because I have been thinking of a treeless saddle because of fit problems on Chivas. The saddle I use has a horn (which I hate!) but fits better than any other saddle I have tried on him.
That being said, it still doesn't fit him completely. It is just a bit low over his whithers, and I have to use a 1" thick pad and a breast collar to keep it in place.
Amazingly, I have two of these saddles, one a bit older, same brand, same style. However, only the older one fits! The newer one doesn't fit right and Chivas won't move out in it as willingly. Weird right? I called Fabtron, the saddle company, and the representative informed me that they have used the same tree ever since they started making that model. The were both purchased used, so I don't know the history.
I even had a custom saddle made for Chivas, sent the company wither & back tracings, yet it doesn't fit. I sent it back, they made some changes, but still it doesn't fit. It's sitting on my sale rack, along with the one Fabtron, a Wintec Pro 2000 and a Wintec Pro Dressage.
Anyway, I have never riden in a treeless, and I have heard bad things about how they hurt the horse's back over the years.
However, in my research, I have read that several Tevis winners and/or finishers rode in treeless saddles. Also I discovered that some of the saddle brands have placed more support in the saddle, and of course the special pads have also been developed to support the backs better.
I am still a bit wary of them, but after discovering several had demo saddle programs for reasonable fees, I have decided to give the Barefoot Tahoe model a try. Hopefully, it will work on Chivas and my other horses, who have way different conformations. The other reason I want one is so I can try Chivas in one of the little local shows in flat-shod racking and I can't ride in a western saddle.
Here's a picture of the one I plan to try.