Brad Marsh has a few screenshots of our upgrade experience from Microsoft CRM 3.0 to 4.0
Summary: it went extremely well.
My comment: CRM 4.0 seems to be slower than 3.0 but since it’s running on an old server, it was expected (new server infrastructure coming in the next few weeks – Yay!). Overall, no real problems. Our CRM scenario is pretty standard with only a few customizations, so we aren’t really representative of companies that have lots of changes.