The new XML/XSL debugging in 2010 makes it a very good upgrade for me. Although it has a VERY long way to go it is still one of the best XML/XSL development enviroments on the market (*which just goes to show how horrible that market really is).
I'm doing Silverlight and a little XNA development at the moment and use 2010 with Blend almost exclusively. I still have 2008 installed but have not had any reason to fire it up recently (that might change if I had to update some of my old projects - I still have some stuff in 2.0). On the whole I have personally found 2010 to be a pretty good upgrade though I will be keeping 2008 aroound for a while, ' just in case' . First impressions of HELP were not good but I tend to use Google/Bing for help nowadays anyway.