How warm is this for under a shell? I am replacing a worn our NF Denali, and would like something at least as warm. I am considering some of the down/synthetic options, such as the MH Lodown or SubZero, but would prefer not to look like the Stay-Puff man.

It is a warm layer. If you are considering a down and you don't want to look like a puff, I would recommend the Patagonia Down Sweater. It is about as thick as this fleece and much warmer. http://www.backcountry.com/store/PAT0359/Patagonia-Down-Sweater-Jacket-Mens.htmlAdded: The Patagonia R2 is not at all like the NF Denali. Firstly, the cut is significantly slimmer. The R2 is cut very slim, and the NF Denali is designed to be an outer layer, or for the larger individual. Secondly, the R2 is a thinner layer. If you want something as warm as the NF Denali go for the more-regular-cut R3 and layer it, or the windproof R4 (which I love) and also layer. Depending on the level of activity, the windproof R4 might be too warm or stuffy. If you're not going to move around a whole lot then DEFINITELY go for a down or synthetic insulation material. The Mountain Hardwear compressors are great and Patagonia have some excellent options in that area too, especially in the down jackets/hoodys...Note to above: the Patagonia down sweater - while thin - is significantly thicker than the R2 and significantly warmer. I would not recommend it for high activity as it will get sweaty and useless *especially* if you're wearing a backpack.