I rode a Specialized Roubaix S-Works SL3 today in the first leg of the 2012 Ride the Rockies. It was a 56 cm unit with Dura Ace Di2.
I think my 2010 Roubaix Expert appears more stable. It may be due to my bicycle having a 58 cm frame or Michelin Race 3 tires vs Specialized tires on the S-Works. I didn't perceive the Dura Ace brakes performed better than the Ultegra brakes. I think the Dura Ace Di2 front derailleur shifts slightly better than the Ultegra Di2. I could tell no difference for the rear derailleur.
I had no trouble climbing with the S-Works. It had a 172.5 mm crank and a 11-28 cassette. Mine has a 175 mm crank and a 12-30 cassette. I think climbs will be easier on mine tomorrow.
The wheels were nearly identical to the Roval wheels on mine. They surely roll well.
That S-Works surely was a great bicycle, but I can't justify an upgrade.