Personally, I think they are great, but they do not suit all horses or all riders, plus all makes differ. I would recommend you try some out before making a decision.
Absolutely the best thing I ever did for my horses' and my own backs. I have dressage one and jumping one. Unfortunately would recommend you go for more expensive model rather than some of the cheapies you see on ebay and such.
Try www.totalsaddlesolutions.com - you can get a trial ride or a week's trial at home - in fact they won't sell you one unless you have tried one. The section on "Why go treeless" makes very interesting reading too, from a horse welfare point of view.
thanks for that im going to look into it a bit more plus she is only 4 yrs old so she will change shape as she gets fitter plus i find the tree saddle restricts her shoulders abit shes such a lump but i love her.