I normally wear a M in Gore cycling gear (I'm 5'9", 180), but this jacket runs quite small. I went with a L and I could fit only a baselayer under it, which is what I was aiming for. The jacket performed really well -- very breathable and warm. I wore only a merino short sleeve shirt under it, and I was plenty warm in temps in the mid-40s on moderate intensity road rides. I can't see using the jacket as a jersey, which is essentially what you get when you remove the zip-off sleeves. If it's warm enough to ditch the sleeves, your core will fry. The zipper is very small and looks pretty cheap, and unfortunately it performs poorly as well. Mine broke after about a month of commuting and "real" rides, and BC stepped up with a replacement, as they always do. Strangely, the replacement was noticealby smaller than the original, so I'm not sure if sizing consistency is a widespread issue. I returned the replacement before using it.