Bought this jacket off of here last year, and it's been one of the best purchases I've ever made. It's seen a lot of use for winter activities (winter hiking, snowshoeing, nordic skiing) in northern New England. I normally keep pretty warm on my own, but I've been able to wear this jacket (with a thermal shirt layered underneath) down to about 0F as long as I've got a good pair of gloves, a hat, and I keep moving. With just a t-shirt, it's probably good down to 20F or so. But be forewarned: it's designed for you to MOVE in it, so these temperature numbers go way up if you aren't moving. I've been cold in this jacket at 45F in the wind when I don't layer and I'm just standing around. But fortunately if you do have to stop, or if that crisp winter day turns blizzardy, this jacket's cut is fitted enough that layering over it isn't a problem.
During the rest of the year, it's a great lightweight softshell to stuff in the daypack to stay warm for a summit lunch. It's also a really good-looking jacket, so I end up using it a lot just around town!
For the price, this was the best cold-weather aerobic-activity jacket I could find. It does what it's designed for, and does it really well! Just don't expect to wear this over a t-shirt on a brisk November day to go sit on shaded, windblown stands to watch a football game. Trust me, I've tried.
For fit, I'm 6', 175 lbs, and the medium was a nice athletic fit that made layering doable.