Parallel libraries - my image processing application works much faster on multi-core machines (4 cores = 1/4 of the time in some cases)
Client profile means I dont have to install .NET 2.0, 3.0, .3.5 and 4.0 on new netbooks or laptops
It would add several months programming were I to regress, but most of the application would have been done in silverlight 4.0 had that been available several months go, so a VS regression is simply not an option.
I tend to rely more on a few good books than the help system, and the VS 2010 training material is pretty good, so the new help is not a deal breaker for me.
If your project does not necessitate an upgrade to visual studio 2010 and you can use VS 2008, then stay with it, its less hassle and cheaper. I have always thought visual studio a bit wasteful in discarding it every 24 months.