LOVE LOVE LOVE this jacket!
I use this for all outdoor aerobic activity. It is ridiculously light. Blocks the wind, blocks the drizzle, and has cool reflectors. I took it on a 30 mile bike ride today in 30 degree weather with a couple midweight smartwool layers underneath and was perfectly comfortable. I also use it for running and it works quite well.
The jacket looks cool too. Get lots of compliments on it's look. It's cut very athletically.
I have about five Arcteryx jackets and this is my favorite.
My ONLY recommendation for improvement would be to make the mp3 player pocket on the sleeve slightly larger. I would typically use my iphone for listening to mp3s and this pocket won't quite hold something that size. Given the prevalence of iphone-sized mp3 players, it just makes sense to have the pocket accommodate something of that size.
For fit, I am 6'2", 210lbs and wear a 42 Long sized suit and the MEDIUM Accelero works perfect. I can fit a couple layers of medium-weight smartwool underneath this jacket all zipped up without issue or tightness. I feel like a large would be a little baggy, which would seem to undermine the purpose of this jacket.