I've ridden this from 50 degrees and pouring rain down to the mid teens. Some reviews claim that this is near waterproof, however, it really isn't. When you first start riding in the rain, the water quickly beads off, however after 10 minutes or so, it starts to soak into the fabric. That being said, it is still way better then being cold and wet with a more standard jersey, or hot and steaming with a PVC rain cape. After one 3 hour ride in pretty nasty wet conditions (upper 40's), my inside jersey was pretty wet, however, I was still warm/comfortable.
As for what it is, its not really a jersey, as its more of a jacket. Where it really shines is for wind resistance. It really cuts the wind keeping you warm, however, it really does breath unlike my older "Tech" coat from 3 years ago which would leave me clammy after anything over 1 hour of hard riding.
In the winter, this jersey/thing is great for layering. For my mid teens (wind chills in the 0-neg 10 range) 2 hour ride, I rode a Helly Hanson Warm base layer (also a great piece of gear) a thin roubaix fleece lined LS jersey and this, and was content. When I finished, feeling all the layers, everything worked. Stayed dry and warm.
Now the fit: its race cut. This is exactly what I was looking for. I wanted a coat that I could actually race in and not be worried about being a parachute. I'm 150 lbs, wear mediums in most bike tops, smalls in most bike jackets, S and XS in most consumer cloths. I bought a medium and it fits pretty much as advertised. Almost tight, yet its flexible enough where you don't feel cramped. I've actually raced probably 10+ late fall races in this jacket and never felt over or under dressed.
I did a lot of reading before I spent the cash on this jacket, but in the end, I'm glad I did. I really like this jersey and ride it all the time.