Cory Akin wrote a review of Arc'teryx Atom LT Insulated Jacket - Men's on March 17, 2013
When I first received this jacket, I was a little disappointed by the lack of , well, jacket. It's a very thin jacket. For it's price you would expect something like the Patagonia Down Sweater, which has substantially more girth.
But, after putting it to use, I am absolutely pleased with my purchase. For being a thin, slim fitting jacket, it is VERY warm. I use it as my insulating piece beneath an Arcteryx Theta SV, and the combination is perfect. It does a great job on colder days when temps are below freezing, although if the sun is out, and temps begin to climb at or slightly above freezing, I found the combination Atom LT/Theta SV was almost too warm. That's a good thing, in my opinion, being as I am confident I won't need any extra layers unless the temp REALLY drops.
For sizing purposes, I am 5'11", weight 180, have a 33-34 in waist, and I went with a Medium. The medium is perfect as a slim fitting, layering piece, although if I were to use this as an around-town jacket, I would go with a large for a more casual look. For reference, I wear a medium in the Patagonia Down Sweater, which is a nice, casual fit. So the Atom LT does fit slimmer.
In conclusion- great jacket, great for layering, very pleased