I got Barefoot Saddle a few weeks ago and I love it. It is VERY comfy! I love how well you can feel the horse move underneath you, it's almost like riding bareback but with a saddle. Barefoot has 2 sizes and they are pretty correct. The saddle molds to you and your horse so I think the "one size fits all" statement is pretty correct. Good luck!
Then I really recommend this saddle. One horse that I ride has a back problem that makes him VERY hard to fit. He diddn't even fit a saddle custom made for him! But my Barefoot Saddle fits him very nicely.
The only thing I have ever heard about Treeless saddles is not good. The Chiropractor for barrel racer Charmayne James said most all chiropractors love the treeless saddles but eventually they will cause back and next problems since there is nothing to take the weight off the horses back at anytime.
I was thinking of getting one, but after that, decided to get a well fitting Billy Cook saddle instead and just love it.
I have a Bob Marshall and a Black Forest (similar to Barefoot). They are great, but they are NOT one-size-fits-all. They also require a good pad to go with them. I use a Skito pad and a ThinLine pad. I'll be getting a Skito Treeless Half pad for the black forest, as it's more affordable ;-).
You also get what you pay for. Buying a cheap treeless saddle is no better than a cheap treed saddle. You want to stick with good brand names like Ansur, Sensation, Barefoot, Black Forest, Startrekk, Freeform, TreeFree, Torsion, Bob Marshall, EZ-Fit Treeless saddles (new brand that looks REALLY cool!), Bandos, and Heather Moffet's Vogue and Fheonix saddles. Stay AWAY from Hilason, Maya, Sydney Saddleworks, Freemax, and anything else that retails or eBay's new for less than $500. These "copy cat" saddles copy the look of quality saddles, but they are often poorly made with internal structures that are not suitable for use.
I am a heavy weight rider (250 lbs), so I use a Skito pad and a ThinLine pad together; on of the simple ThinLine Western pads. The extra pad under the Skito provides more weight distribution and is VERY non-slip. I feel quite secure in my saddle, even on green horses or "fun" trail rides. I use a 100% mohair roper cinch for added stability. For trail riding, I use an over-the-neck aussie/english style breastcollar (we have lots of hills!).
Here's my set up.
I swapped out the regular stirrups for padded, caged endurance stirrups. I have no problems keeping my leg under me for a "classical" position.