I'm 5'9 and 165 and bought Med. They fit tight, but not restrictive. The straps are great and lay flat. The coverage comes up high on the sides and about belly button height on the front, which suits me. The vertical leg grippers are awesome and do their job well, even if you're not a leg-shaver. The fabrics are thicker than usual, but they are quality. I think these would be hot to ride in above 80 or so.
The big disappointment is the pad. Yes, it's good for Castelli, but not a great value for the price. It floats on it's own panel which is kinda cool, but it isn't really better than bibs that don't have this design. The pad is dense, but for me, it's a 40 mile pad at best.