The Atom LT is a great jacket and a really superb layering tool. Arc'teryx proves once again that their stuff really can't be beat, as this has quickly became one of my favorite layers I've owned. The athletic fit really helps with fitting under a shell or heavier insulating layer, as well as having a better fit for those of us with slimmer builds. Synthetic insulation is also key for the Pacific NW, performs really well under the wet conditions that you find around here.
I'm 5'8" with a slim build and a small fits perfect.