I've been searching around for the right shoulder season jacket to keep me riding a bit further into the fall and earlier in the spring out here in Utah. The tough part for me was finding something that would work for MTB, road cycling as well as any other aerobic outdoor exercises. Personally, I think that this jacket was the perfect find.
The zip off sleeves make it super versatile for road and mtb. There are two zips per sleeve, so it is a bit of a task to get them back on, but nothing seriously bad. The windproofing of Gore is top notch, and though I haven't been out with it in a full down pour, the water beads right up and runs off in the slight shower it has been in. Oh, and the breathability is excellent, hasn't felt muggy or sticky at all.
NOTE, I am 5'8 and about 155lbs and the large fit me well for what I was looking for (not ultra slim, since I would be MTB in it). I can put a long sleeve base layer under it, but not much more, so I dont think I would have gone with a Med. Hope that helps...