Well my Ion arrived (Marmot, not OR), and it really runs small. Im 5`9.5" and 165lbs. The waist line is high, at the belt line, and it is cut really trim. Im exchanging it for a L so I can layer it over a 100wt fleece. I suppose the cut is just what a runner would want, as mentioned in other posts, but I need more room for this backpacking application.
As for the features of the jacket, I like the full zip (unlike the OR ion), and having a pocket is useful. Interestingly, the weight on my M was 3.3oz, not 3 oz as advertized. Maybe 3 is for the S.
Ill edit this post after I evaluate the performance of the Large.