I purchased this jacket in Large about a month ago and used it quite a bit so far. Regarding sizing, it is a slim / athletic fit and I went with a large. It proved to be more trim fitting than my North Face jackets (Wind Wall fleece and Redpoint primaloft).
The arms have an extra 1/2 to 1" but it appears they are cut that way so they don't ride up. Overall I found it a super light jacket (1 lb, 1 oz) and I used it over just a base layer down into the 30s on a short trip recently.
The price was right!