LOVE them, they're the only saddles I use at this point. The sizing is different, the size go based off of your thigh measurement. It explains in on their website: Sports Saddle, Inc. :: Bob Marshall Treeless Saddles
Biggest con I've found is that if you have a horse with a very weak back or really prominent spine, it might not be the best choice. Biggest pro is that it's REALLY comfortable, and I can't trail ride in a regular treed western anymore, it's just...ouch. I like that it fits every horse that I've had, and they all do quite well in it.
You want the Bob Marshall maker, not the BM by Circle Y. Also, they can roll a little bit on certain horses, so I use a wide cinch, either neoprene or wool, and don't have any other problems.