Its best application is mountain-type hunting; I regret I didn't have it in '05 when on Kodiak Is. chasing Sitka deer. I could've used that gear only and lightened my load substantially. I've used it for about 50 straight hunts here in Nebraska that were mostly treestand sits.
I think it's GREAT stuff. As mentioned before, the system alone is not warm enough for COLD (20's and less) weather stand sitting. I get cold easily, but have found a layer of Winona sandwiched in between the inner next-to-skin layers and the vest/90% coat works great. Best thing is the 90% coat (order the larger size if undecided) works great as outer layer. The sleeves are tapered so many have said they don't need an arm guard.
Biggie's right, the little silicone strip around the inside of the pants is a huge feature. Most of the items feature 4-way stretch and that's a nice bonus when trudging along with pockets full through the brush, or when trying to climb your tree.
The 90% gear is darn near completely waterproof. I sat for three hours in a moderate rain and never felt a drop or leak. Also like Biggie, I think the camo pattern is very good. In many ways, I think it (Mountain Mimicry) may be the most VERSATILE camo pattern out there. I've hunted cornfields, cedars, hardwoods, from the ground, in light snow, and never felt like I wasn't hidden. They also make it in ASAT which is another camo many feel is very versatile.
This versatility is important because Sitka Gear isn't cheap. But when the camo is so universal, you don't need so many different pieces. And this stuff is built to last.
Another thing, on the 90% coat/pants and the Mountain Pants...burrs either don't adhere or if they do, they are easily removed. Where I hunt, the sticktites, beggar's lice, sandburrs, foxtail, etc. are really bothersome. Wearing this stuff lets me hunt the thick stuff without ruining my clothing. (That said, the Mountain Shirt DOES pick up burrs, but I usually have the 90% coat on anyway so that's not an issue).