Replacing my XTR cranks with the S-Works Carbon Cranks saved ~71 grams. Following the directions and recommendation of Specialized I opted to install the bearing sleeve (if you decide to leave it out you'll save an additional 30grams).
If I ever decide to go to a 2x crank, and save even more weight, Specialized also sells just the spider.
The new cranks were professionally installed by Ryan at Mountain Bike Specialist. Ryan is Todd Wells' Durango Mechanic and has seen the only issue with the S-Works cranks that I know about. Todd had an internal crank nut come loose. To prevent this see "Installing the Carbon Crank" step 2, make sure you add loctite to #9- Steel Retainer Nut as shown. Ryan used red loctite on this one place for me since I was overly paranoid. I recommend using blue or green loctite everywhere else as recommended.I wear all cranks like this, my XTRs look the same way. I should have used a crank protector.
For the first couple weeks my chain would stick to the chainrings, occasionally to the point of getting chainsuck. I had the same issue using a (6 week old) used chain and a brand new chain. Both Ryan and Specialized recommended running the cranks (with either chain) a few weeks to see if the chainrings would wear-in, they did. It took up to two weeks and I no longer have any chain sticking issues. I would recommend allowing 2-3 weeks before a big event in case you need this break-in period.
I think these cranks spin more freely than the XTR cranks did. (The XTR cranks were installed with the S-Works bearing adapters, not the XTR bearings.)
Bottom Line: Buy these cranks for the weight savings, bling, 2x option, and stiffness (although I can't tell the difference in stiffness). Remember to follow installation directions. If you are picky you may notice less smooth shifting vs. XTR.