It's shockingly warm considering it weigh so little and compresses down to less than a 500ml Nalgene. I use it all the time in the city and in the mountains, so I'm getting my money's worth out of it.

I took it ice climbing in February, and even though I had my puffy down belay jacket, I stuck the Patagonia jacket in the hood of my pack to either wear on it's own (it was about 0C) or as an extra layer under my belay jacket (I can get cold!!). At under 300g, no big burden.

It fits a little big, so I wish I'd got a small instead of the medium, but that won't be bad this summer when it becomes my backpacking insulating layer and I might want to get a fleece under it.

I replaced my MEC Northern Lights with this jacket, and other than having to take greater care to keep it dry, I like it much better thanks to it being both warmer and lighter. (Though to be fair, five years of ABUSE of my northern Lights has left it pretty much loft-less)

One minor nitpick is the lack of two way zipper for belaying.

Update at Aug 2008:

Well, this is an amazing jacket. Less than 300g means it is almost always in my pack, and has provided real warmth when needed, and for so little weight. It supposedly has a DWR, but if it's raining, get a hardshell over this jacket, fast! I've used it backpacking, climbing, in the city, you name it.

Update at August 2011:

Still an essential go-to piece of my clothing system. It's been ripped, melted, and the shell is repaired in many places. But, it still has tons of loft thanks to the original high quality down. These are not inexpensive jackets, and the shell is a little fragile, but what you're buying is great down. Keep the shell intact (or keep fixing it) and it keeps you warm without weighing you down. Recently, on the summit of Mt. Olive, I popped it out of the lid of my pack, and presto, instant warmth.