I do a lot of winter running in the snow, in 0 to 30 degF temperatures, and typically wear a breathable windproof jacket on top of a short- or long-sleeve tech shirt. For years, I've worn a Mountain Hardwear Transition jacket (ground-breaking, now discontinued) and its successor, the MH Effusion (lighter, more stretch, but not as breathable).
When the BD Flow State Jacket arrived (in bright red "Torch"), my first reaction was that the color was spectacular -- really rich and bright -- the jacket is truly beautiful. My second reaction was that it was a little more heavyweight than I had hoped for. This is still a lightweight shell, but not a featherweight garment.
Took a while for temperatures to drop low enough for me to start using this regularly. (Abnormally warm December in Utah.) When I finally did, however, I was blown away. The breathability is awesome. Probably the most breathable windproof jacket I've ever worn.
Otherwise, there's really not much to complain about. The jacket has two front hand pockets that I didn't think I wanted, but yesterday I went for a run with damp gloves and was very happy to be able to stick my hands inside to warm them up. The fabric is nicely stretchy (always a plus with a form-fitting garment), and feels like it should be durable.
The fit is definitely slim and "athletic": I'm 5'11, 155 lbs, 40" chest, and the medium is perfect for me. But perfect for something close-fitting: there's just about room for a lightweight fleece underneath, not much more. If you want a more casual fit, or space for more layers, then you should probably size up. Or maybe just look for a different jacket: this is definitely optimized for high-intensity activities.