Why wear a base layer? It's all about comfort. Even the most high-tech, form-fitting jersey may not wick sweat away from your skin as fast as the lighter, thinner materials used in garments like the Gore Bike Wear Sleeveless Base Layer. No matter what you wear it under, it's an affordable way to stay as dry as possible on long rides, or to minimize chafing from bib straps or other agitators. You may even be able to stand a few more miles than usual before calling it quits.
The Sleeveless Base Layer is meant to fit like second skin, and its polypropylene/elastane blend provides exceptional moisture management. The mechanical push-pull effect of the fabric sucks sweat away from your skin to the outer layer, where it can evaporate. Flat-lock seams enable the base layer to glide over your skin. And speaking of seams -- there aren't many to be found. Gore Bike Wear made the back extra-long to cover your entire lumbar section. The Gore Bike Wear Sleeveless Base Layer is tight-fitting and comes in sizes Small through XX-Large. It's available in Black or White.
I got this singlet for the cool morning rides (60-70 deg), but I've continued to wear it in temps up to the low-80s. Gore's sizing chart worked perfectly for me. I'm 6'0", 183 lb. In most brands I'm a Medium, but in Castelli and Gore I'm a Large. The fit of the Large is perfect for me: snug without grabbing anywhere. It's so light that I don't even notice it once I'm on the bike.
So I ordered some of these in XL and XXL - my chest is 41.5". The XL was snugger which I initially liked although the XXL also fit well. They feel great wearing under a dress shirt (I know, hold on a sec there). Then the weekend came and I wore them under a jersey while riding - not comfortable at all. They are too tight on the shoulders and neck, giving a similar sensation to bib straps that are too short. Also, the arm holes are too small - they tug at the pits when in a riding position. They wick well and dry quick when air can hit them - on the lower back under bibs though - it stayed nice and wet.
All in all, I'll stick with the LG carbon ones - much, much nicer and better overall fit (although they snag easier).