I bet this has been beat to death.. but..
I love treeless saddles. For some people they don't work. Simply because they are bad riders, don't balance, et. That's where the damage comes in. But you can get that in an ill fitting treed saddle.
They require pads that give better ventilation and sweat relief. I use a plain old felt pad. Works wonders. I am now adding a thinline as well.
Treeless saddles come in all price ranges, qualities and styles.
There are "true" treeless saddle (such as the ansur, sensation, and cashel) and there are the semi-treeless saddles (such as the bob marshels, might be others but can't think of them).
Semi-treeless will have a hardened cantle and pommel. But the only thing that connects them is the padding, leather, rigging of the saddle.
True treeless is more of a glorified bareback pad but with a lot
more grip and security. Which is why you can use stirrups with them. But since they have no ridigid parts they typically have a weight limit as well.
You can find really cheap non brand saddles that run about $100. My boarder purchased such a one and after replacing the cheap, cheap, cheap nylon stirrups with stirrups from her old wintec western saddle, it is doing fine for her leisurely trail rides. Though she switches saddle pads like nobodys business.. makes me wonder.
You can also find them brand new in the $3500 and up range. Built with what ever rigging, stirrup placement et that you want.
I went the middle route and found a used one ($500). Its a bob marshall sports saddle and it is OLD. Very old. I think it was the original sports saddle before bob marshall came into the scene. But not positive. Only thing that needs fixed on it is the stirrup leather covers. The velco has gotten very old and just doesnt velco anymore!
I find mine to be the ultimate in comfort for me and have never had a hrose reject it. From a high withered tb/qh mare to my dainty arabs, to braod backed qh's to a pony! A tall pony since the cinch rigging hangs so low I would need about a 5" girth to fit anything smaller!
I find them easier to fit to a horse then a treed saddle, more verstaile, and just plain comfy. Plus I cannot ride in a treed saddle. I can't physically sit in one with out being in pain after about 10 mins. Old injuries.
Here are some pics..
My 14.3 850lb 20 yr old arab gelding. Narrow withered broad back.
My 14.1 900lb 12 yr arab mare. Dainty. But deep chested.
My 15.3 1100lb 10 yr old arab/paint gelding
None of them have a problem with it and garunteed that each has a different back.
I like the deeper cantle and pommel becausemy horses do stupid things and we go over some rough trail. Its nice to be able to be able to grab something or stick my butt against. Like leaping 10' in the air over a 2" creek of mud....
But that's my opinion on it and I love them. And my horses sure don't mind them.