I wouldn't say it packs to the size of a nalgene, more like a Nerf football-sized. Still pretty small though.
The only catch to it it is , I'm 6'4" and I just barely fit in the Long version, so if you're taller, you won't fit. Also, this is definitely a "summer only" bag. The insulation is practically non-existent, but it fits tight and traps a fair amount of heat.
Like I said, I've used it to the upper 30s with long johns and a beanie, but that was one fluke night during a long trip in Northern Minnesota. I wouldn't recommend it if the lows are predicted to be below 45° for consecutive nights.