I've been playing with VSTO in VS2005 beta 2 today and I can't help but say how impressed I am with the integration capabilities and how easy it is to use.

I've always thought office integration was a bit "mickey mouse", but the new version has opened my eyes to lots of possibilities and ways to make things so much simpler for end users that I'm actually quite excited about it.

For more info on VSTO have a look at a few resources:
Code Snippets for VSTO
VSTO Blog