First the fabric - really nice weight Merino. Not too thin, and very even, well-knit fabric. Merino is expensive, and it seems like many brands are experimenting with ways to blend polys and other fibers to offset the Merino cost (and downgrade the overall fabric). Not Wooly. The clothing really is fabric-first, 100% merino, in a weave I suspect will last well. I have a well used henley of theirs that has stood the test of time, and I expect this button up to do the same.
Fit - perfect. Medium is a true Medium, but isn't constrained. Plenty of room to be comfortable and look great (and hide a little of my middle!).
I'm 5'9", 32 Waist, 175# and the medium fits me perfect.