I know it's a roadie question, but I trust your guys' opinion 
My road bike is an all ultegra sl group except the cassette (105). So I've got a couple questions, why would I want to (should I?) upgrade the 105 cassette down the road? If so, look at matching with the rest of the ultegra sl group or just jump to the dura ace cassette?
I'm bored @ work and making a "what I'd like to buy for my bikes" list so this question came up. 
The Ultegra SL is a really fine groupo. Dura-Ace is the pro level and not too much distinction between that and the Ultegra other than cost. Yep, slightly lighter; yep, bearings and such a bit different. If you look at the road groups like you do the mountain groups where in mountain you have LX, XT, then XTR in the hierarchy; in road you have 105, Ultegra, then Dura-Ace. Some may disagree because it's often all about "what you got foo", but 105's are good quality components. Ultegra is a bit smoother and quieter and perhaps a bit longer lasting too. Dura-Ace is a good smooth groupo to have if you're looking to really shave weight, but remember in order to make any difference with that aspect you need to do it across the board, in other words not just one or two components or it's a waste of your money. So if you swap cassette and deraillieur lets say for Dura-Ace - mostly your getting the benefits of just looks in the grand scheme of things. It is understood to go for less spinning weight and the ultegra cassette will do that for you without breaking the bank. That's not to say the 105 is a bad choice, for it is a good quality cassette, just as the LX cassette is my choice on the mountain bike. And yes, on one MTB I have XT deraillieur, on the other I have XTR, and on my hardtail it's all LX and great fast ride. My road bike has Dura-Ace because that's what came on it and it was a bargain. My last road bike was full Ultegra and I loved every one of the 6000 miles I put on that bike. The short answer - 105 until it wears out, then for more competitive performance replace with Ultegra, unless your dollars are more free then by all means go full blown.
:-)
