Using 11 now, for about 1 week, it's pretty much the same as 10, and is able to pull in the 10 files without much headache. They did improve the filleting capabilities, but it still lags behind SolidWorks imo. For me, Inventor 10 was occasionally crashing (although it would usually do so politely, and allow me to save & exit before fully augering in) when I had too many complex models and/or assemblies open, and then tried to create a drawing. Supposedly, this should happen less with I11, so I went ahead and installed the new version. I'll let you know if there are any major headaches in the next few weeks.
The biggest tooth-gnashing is coming from across the aisle, my colleague installed the new Vault version along with Inventor 11. The new Vault version doesn't support user-defined content center libraries in the same way as it did with Inventor 10, so he has no links to his content center files. A bit of a pain.
I never really use the Vault, so it was no big deal to me.