I put all XT 10sp on my old crappy Trek and couldn't be happier. It was about 5 years ago now and it still rides like it is new, with fast, one finger shifts up and down the entire cassette. This replaced old 7sp LX I believe that worked but not nearly as well. To go with 10sp though I needed shifters, brake levers, new 10sp wheel, cassette and chain. It was the entire system not just the derailleur as 10sp Dynasys is a different beast than 9sp. I also put XT v-brakes on since I needed separate levers anyway. But, having said that, it is a slippery slope in that one change leads to another then another. All told, that part of the upgrade was $330 which included a 10sp wheel. The other part of the upgrade was the front dynamo wheel and that was another 130 plus 110 for the light itself. So I'm almost $600 in on an old bike.

So while I've done it, I also could afford it. If buying a $70 derailleur is a stretch for you don't do it.