This is a simply fantastic piece of gear. Not only is it almost magically breathable, weather-resistant, lightweight, and non-restrictive, but it looks great, too. When I go hiking or backpacking in any season but summer, it's pretty much always on. I keep a fleece around for extra warmth if I need it, and a hardshell in case the weather gets really nasty, but they tend to stay in my pack.
However, despite my undying love, there are some caveats to consider before you drop more than three bills on this wonderful, wonderful jacket.
First off, it fits very close. If you're carrying more than a few extra pounds, it's probably not for you. I've got a slightly, uh, "stocky" build (5'10", 165 lbs, 34" waist), and I'm right at the edge of it looking good on me. If you've got a belly, it's going to look ridiculous, sorry.
The close fit also means that layering insulation underneath it can be annoying. Arc'teryx uses a pretty unique cut, that's great for the athletic body type, but also a lot more form-fitting than even other gear manufacturers. Stuff that doesn't fit close is going to bunch up. It's not a fatal flaw, and living with it isn't as bad as you might think, but it's worth considering. You may want to try to find an outdoor retailer near you who at least carries some Arc' products, so you can get an idea of how their stuff fits.
Finally, it's short in the front. Very short - I've got a medium Gamma MX, and a medium American Apparel t-shirt pokes out of the bottom a bit in front. It comes to maybe an inch below my beltline. This is great in terms of keeping maximum freedom for your legs when you're out in the wild, but if you're looking for a casual jacket, it's worth keeping in mind. It does tend to make you look a lot taller (woohoo!) but if you want to wear it with casual clothes you're probably going to need to tuck things in so they don't look ridiculous.
As far as weather resistance is concerned, you should know that, while it's good, it's no Pro Shell hardshell. Wind goes through a little bit, and while I was comfortable yesterday with just a base layer and the jacket in 30 mph steady winds with maybe 50 mph gusts, I wouldn't want to be without something totally windproof in my pack just in case. Similarly, while it's more than capable of shedding light rain and surprisingly comfortable even when things get wet, I wouldn't want to get caught in a downpour without something actually waterproof. This is a supremely breathable and comfortable jacket for 90% of conditions, but you should know about that 10% it can't handle.
Despite all that, though, it's incredible, and amazingly, worth every cent. Buy it; it'll cost you a pretty penny, but it's great nonetheless.