I have used the jacket on quite a few rides since Christmas, with temp 2'C-15'C, and it is very very good. Wind block is great, and the zip front and chest zips help relieve overheating on the hills. As noted in some of the reviews the jacket is quite a snug fit.
I only wear an Assoss thermal under (either short or long sleeve), and it's about all that will fit and all that you really need. I haven't been cold yet. I usually wear medium, which I bought in this jacket and it fits very snuggly.
You would not want to be a big guy. I am very lean, 5'11" (180cm), weigh about 68kg with chest 96cm (36"-38") and waist 82cm-85cm (32") and you would definitely not want to be any bigger for a size medium.
It is a very good fit if you want a snug jacket, though I have found the neck a little too snug at times, but lowering the zipper seems to sort it out when you warm up. The quality is very very good (as is any Castelli gear I have bought) and it looks very good (which is all that really matters!!!!)"