If you're looking for a super light weight rain shell to take for a speed or endurance assault on pretty much anything, this jacket is it- it's simply amazing. I bought it as strictly a super light rain shell for endurance hiking but have found its usefulness has far exceeded this, proving valuable as a wind shell as well.
Firstly the weight will blow you away. 5.5oz is amazing to behold and then to lift. It also packs away very easily into small trail running type packs.
Beyond the weight is its performance. Even though I have other rain shells I almost always wear this. It wicks reasonably well but more importantly is completely waterproof. I have been hiking in downpours and this has completely saved me (or at least my upper body).
Combine this jacket with a nice insulation layer (I use the Patagonia Down Sweater) and your upper body is ready for near anything you'll get in the mountains (or at least the Whites in the summer/fall) while not even breaking 20oz (!) in weight. This simply blows my mind.
I've only had it for about four months but have used it quite a bit and haven't really seen any signs of wear. We'll see one year on, as it simply doesn't seem like it's believable that such a lightweight performance layer can last regular use. I guess that's my only caveat so far.
While it's a bit pricey for a student such as myself there really doesn't seem to be anything that competes with it. I haven't regretted putting the money out for this at all. If you're on the fence, go for it!