I only know what I have read, and heard briefly from a friend of a friend, so don't take my word for it. For trail riding, over reasonable terrain, I would think they would be great - and for horses with very wide backs, or with odd fitting needs, they may be the only option. For extreme mountain trails, for working cattle, or any other riding that needed fast movement or abrupt stops, turns or inclines, I might think twice. I don't know that I would want to jump in one, but might be persuaded, certainly over lower jumps, up to 3'. For riding many different horses, of all sizes and shapes, I would think they could be a Godsend, as long as the horses were reasonably well broke. I don't think a treeless saddle would be my first choice for a broncho-busting session!

As far as showing, I guess it depends on the type of show - some breeds or divisions have strict regulations that a treeless saddle may contravene, and some have even stricter, unwritten, fashion rules - it may not be AGAINST the written regs, but the judges may pass you over for someone who is as good, but who followed the tacit regs..., esp. if the saddle LOOKED different from a treed saddle. Check with the rule book for your desired discipline, and maybe talk to a few trainers or competitiors, some of whom might be more helpful than others. If winning a ribbon is all important to you, it might be better to stick with the familiar.

I seem to recall that either The Horse Journal or John Lyon's Perfect Horse did an article on them not too long ago, so you might try googling, and see what comes up.