Arc'teryx Men's Atom LT Jacket

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

This jacket receives my praise on so many accounts that even at full price, it feels like a bargain.
To better understand my use of the jacket, it may be worth noting that I live in a coastal desert region. My need for a jacket is limited to cold nights and mornings, a few short winter months, and travel to colder regions. That being said, the Atom LT is light, compressible and breathable, which warrants having it on me much longer than I would a heavier jacket. This is one of the main reasons for my purchase.
Others have mentioned that the piece does not appear very "technical" while still offering great performance. This jacket suits a casual setting just fine. Not everyone will care about that, but kudos to the bird for designing a jacket that excels in both performance and aesthetics.
Lastly, I am probably one of Arc's huskier customers, and I don't always get along with clothing labeled "athletic fit." The XL fits me JUST right and with excellent range of motion. It is like wearing nothing, and I don't look like the Stay Puft Marshmallow Man, which is a good thing..
Money well spent.

I've owned this jacket just over a year. I wear it everyday from October -March. It is the perfect piece for cold commuting on a bicycle and chilly days to the craig.
I was very skeptical of the Coreloft insulation before I had purchased the jacket, and now I am a believer. It regulates with your body temperature very well. Not quite as good as premium down, but it still works amazingly.
Thinking back on it, I wish I would have gotten the hooded version, but this piece is very nice, but it also passes for looking like casual jacket.
Don't get me wrong, I'm a huge gear whore, but I don't feel the need to have every jacket look like I'm geared up to Summit K2 when I'm going out to dinner.
For sizing, I'm a pretty thin guy. 6'0, 135lbs with an even ape index. And I think it fits very nicely. It's hard to find a medium that actually fits these days.
The only thing I would add would be elastic waist tensioner so the bottom was more form fitting to my actual waist.

Read the reviews. This jacket is awesome. I am not going to give you all the details. You will find them in the glowing reviews listed here and on other websites. I am up to my eyeballs in Arcteryx & Mammut jackets because they are the best overall quality. The Arcteryx Atom lt. is my all time favorite jacket. It is incredibly warm and light and looks outstanding. I have worn this jacket all winter by itself and as layer underneath a gore-shell jacket. It is a very versatile jacket for the spring, fall, and winter seasons. I am an athletic 5'9" at 160lbs and a large fits nice with room for a layer underneath. At the risk of sounding obsessed, I bought the hoody and vest version as well. I love this jacket. It is extremely versatile, light, warm, somewhat breathable, wind & water resistant. PS I bought the vest in medium for snugger fit.

I bought this jacket in 2009 and it remains one of my favorites. It feels great over a wide range of temperatures, say, down to the 20's (if you're active) and up into the 60's. It's light (my medium weighs 12.3 oz) and the trim fit looks great around town AND on the trail. The "Powerstretch panels" do a nice job with ventilation during exertion. Jacket does have an internal chest pocket which is not listed in Product Description above. There's no cinch at the waist hem, but it's stretchy and wind-up-my-back hasn't been a problem. I'm a big fan of the "stretch-knit cuffs" - Arc'teryx chose a sweet fabric and nifty design for this part of the jacket (and the cuffs just feel great on my wrists). I can't recommend this jacket enough.
Pic below shows Atom LT in spitting snow at 11,000 ft in the Wind River Range.

I've ordered, and returned, two of these jackets so far.
The good: They are fantastic in build quality and fit, and in short trials walking the dog (tags still on) they are very warm. I'm looking forward to getting my third one.
The bad: the color pictures on the website (and on Arc'teryx's website) are not accurate indications of the color of the jacket. For example: Squid Ink, which looks to be a nice blue/periwinkle, is actually a deep purple; Tibetan Red, which looks like a nice dark red, is also more purple than red, with a ridiculous light orange lining.
Not being man enough to pull either of those colors off, I'll be placing my my next order for the black version. Hopefully the third time will be the charm.