I had a chance to try out the Ion Sunday morning on a training hike in the local foothills. Unfortunately, the day heated up quickly and I was only able to test the windshirt for an hour before packing it away. The test conditions refer only to that hour.
Temps were mid-thirties to mid-forties with a steady cool breeze. I was wearing a thin Golite C-thru base layer under the Ion and carrying a Jam pack with 15 lbs. of gear. For part of the test I was wearing a one ounce skull cap. Later I switched to a sun hat. The Ion was worn in configurations varying from fully zipped up with the hood closed around my face to fully zipped down with no hood. My heart rate ranged from 120 to 140 BPM.
During the test I checked frequently inside the jacket and could detect no condensation. So for me, at least, the Ion is adequately breathable.
Based on the fabric, fit, features , and quality of construction, the OR Ion easily displaces the Montane Featherlite smock that has long had a place in my kit. The 14" no-snag zip and brimmed, adjustable hood more than make up for the additional 0.3 oz.
I just noticed that one of the reviewers I linked to above is our own BPL member, Mary Simpson, who gives the Ion five out of five stars based on a hike in the Grand Canyon that was partly off trail.
For $40 at REI-outlet.com, the OR Ion is probably one the the best values out there right now.