Outdoor Research makes some phenomenal products, and you know with a name like Deviator it's gonna be good. OK, maybe not as good as if it was named Deviant, but I know they're walking a fine line there, corporate social responsibility and all that.
The Deviator is a class of clothing that didn't exist until this past decade: a light insulation vest with light to medium weight technical fleece style sleeves, and sometimes a hood. The format has proven to be insanely useful for everything from layering to light and fast shoulder season work. The Deviator is beautifully designed for its own niche, which is running, biking or anything fast, but it also is a stellar performer for high-output backpacking and hiking.
The trick is that the state-of-the-art Polartec Alpha synthetic insulation is only in the front of the torso area. The back, sleeves and hood are all Polartec Power Grid fleece. This combination ensures that your core stays toasty even at speed, but you still have wicking and breathability everywhere else to ensure that you don't overheat. Not having insulation in back also makes it great for backpacking, since you've already got a heater strapped to your back.
When you stop to catch your breath, pop up the hood for a little extra warmth. A little cold at night? The Deviator works great inside a sleeping bag too. It's a lightweight versatile piece that works just about everywhere, and OR didn't skimp on the features: open handwarmer pockets that are deep enough to keep stuff in, a zipped chest pocket, hem drawcord, and a full length zipper flap with neck protector at the top. The Deviator is cut close, so consider going up a size if you're normally on the border between sizes.