I just purchased/received an Overland jacket from an inmate here in the Flea Market and have been watching this thread for a while, hoping to contribute a bit so here it goes…
Took the jacket out for an hour or so ride last night in about 82º / mild-humidity with a cotton t-shirt underneath. The pit-zips and exhausts do their job to keep your core cool, however some fore-arm vents would be nice. This wasn't technical riding by any means but I can see how doing some trail riding in this jacket you'd work up a good sweat. I'm not going to get into the breathability of the Gore-Tex because that's probably been beaten to death in some other Klim thread. It is what it is.
As some have stated the D3O in the shoulders and elbows is a bit thin, but the user can upgrade if they feel necessary.
Cut and patterning of the jacket is nicely done. It's obviously a more entry-level piece of gear but is still nicely patterned with good ergo's and sensible, thought out feature placement. I'm a lanky dude — about 6', 165lbs and find the medium to be a surprisingly perfect fit. It'll allow for layering from fleece to a low-fill down insulating jacket. Plenty of room for a pressure suit through the shoulders and sleeves.
Hope this helps anyone considering the jacket. FWIW, YMMV.