I've run BCA packs, ABS packs and now the BCA Mountain Pro Vest.
First thing I noticed from going from an avy pack / tekvest combo to the mountain pro vest was how much cooler you run!! Not having the tekvest against you with a coat over the top and then the pack on your back really changes the "warmth" factor. It's not bad, its just different and you have to readjust your layers and learn how to dress again, if that makes sense?
I think you end up with a little more "freedom" of movement in the mountain pro vest as it fits nice and having the weight of the pack distributed across your entire upper body is nice. Instead of the weight pulling on your shoulders and sitting on your upper back, it really is dispersed through the vest.
The mountain pro is smaller as far as storage then my previous packs but I don't pack a ton in the packs, so that wasn't a huge deal.
The pockets on the mountain pro are nice with having special pockets for the link radio and your beacon. Getting to the beacon is super quick and easy, no unzipping a coat and getting to a harness. It's also nice having one piece of gear that basically replaces two pieces of gear.
As far as negative........I really miss the ABS system, as they are great and don't leak!!!!! The mountain pro also has less or I guess you could say not quite the protection that a tekvest offers. There basically thinner and I don't believe they would absorb the same hit as a tekvest. I have also seen the main zipper on the mountain pro vests take a crap too, which sucks!!! No zippers on a tekvest and their pretty much bulletproof.
I have one season on my mountain pro vest and all in all I'm happy with it. I would like to see the new version and maybe upgrade again, but like everything else, I'm sure they will keep getting better and better!!