Im 5'-9" and just under 150lbs. I ordered the Medium, and without body armor, fits like a dream. The material is amazing, and by far the most comfortable jersey I have owned. I have the Troy Lee "Rincon" Body Armor, and because of the material of the jersey, it stretches over it like a football jersey. Even in the M, it works with body armor, as you cant tell any restriction or additional tightness other than the armor jacket itself. If you are planning on wearing it with armor as a general rule, you might want to go up a size if you like a little more slack in the fit as I do.
BTW - I ordered on Wednesday morning, and it was at my door by 2pm Friday with Huck's free shipping - Outstanding service Huck!