Exactly what I was looking for...
This jacket is a great all-around winter jacket for what I would call a "normal" winter climate. I'm a mid-Atlantic rider/racer, so winter training temperatures generally stay around the mid-30s. This past weekend the ride started around 35 and got up the the low 40s. I wore the jacket with only a "cold gear" thermal base layer underneath. (I also had a summer-weight short sleeve jersey, but just for the extra pockets.)
The Power SO jacket does have three normal pockets like most jerseys, which was a big appeal to me. I generally ride with snacks on the outside and a third bottle in the middle - this works well here. The pockets sit at the correct level and are easy to access. The majority of the fabric is windproof.
There are small, more breathable patches of thermal fabric on the upper back (between the shoulder blades) and on the front of the elbows for flexibility. The back breathes well, although I was surprised how much wind gets through the breathable elbow panels. Took some getting used to, but I do not mind it.
I am 5' 7.5" and under 150 pounds and purchased the size small. The jacket is fitted well, but not constricting whatsoever even with a thick baselayer and jersey underneath. The shoulders and sleeves provide space without flapping in the wind. I'm not too tall, so the sleeves are probably a half-inch too long for me. Not an issue. When standing, the jacket seems slightly long for me too, but on the bike as it's intended it fits much better - the back is longer to cover your entire jersey and a bit more. It's not too long to get in the way of the saddle, though. The front of the jacket has minimal bulging of fabric when in the drops, at least I haven't been bothered by it.
I got the jacket in black, which looks great and generic to match any riding gear. There are small reflective strips, but very subtle.
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Castelli windproof jackets in size small are probably 1/2 size smaller than this Gore. The length was okay for me, but the fit overall was tighter. The Castelli I've used also had no breathable panels, so I would get very sweaty if I didn't open the jacket. I replaced my castelli because the zipper broke OFF.
I also tried a Capo Verona jacket in size small but returned it. First of all, it was too long for me in the torso (even longer than this Gore). Mostly, though, it was VERY snug in the shoulders/armpits and arms so wearing with anything more than a sleeveless base would be constricting. The arms were even longer than this Gore, too.