I've seen a few up close comparisons of 7800 vs 6600 vs 6600 SL. It comes down to fit and finish and very minor weight savings for a lot of extra money. For example the DA derailleur has bearings for both pulleys whereas the 6600 only has bearings on one. If money is no object go DA but the reviews I've seen (and most riders) can't tell the difference when riding on the road, so it's really for show unless you're a top level professional and a few grams really could make the difference between winning and coming second.

On the down side some DA components wear much faster, cassettes in particular seem to have a much shorter life than their Ultegra counterparts. For my money I generally stick with Ultegra although I do have a nice set of SL-7900 DA shift levers for my vintage bike because they're shiny and look pretty (and have the words Dura Ace on them :-)). I couldn't resist when PBK had them on sale for $33 recently which was half what I paid for the 8 speed SL-R400 levers I was using.