So far the upgrade has been going very well. I'm very impressed by solutions in CRM 2011 compared to the quirky things I had to do to get my entities moved across organizations in CRM 4 (We have about 250 custom entities - with many relationships between many of these entities).
While migrating our development instance recently we were hitting the error
"The custom activity does not contain a valid CrmWorkflowActivity attribute."during the UpgadeWorklowActivity step (near the end of the upgrade). Using Process Monitor from System Internals I was able to track it down to a failure just after trying to load the Microsoft.Crm.Sdk.dll (version 4.0). I had deployed the aseembly to the Server\bin\assembly folder with my CustomWorkflow library assembly.
Seeing this, I tried several things (removing the DLL, gacutil, etc). Each time waiting 1 1/2-2 hours for the migration to fail.
Frustration was kicking in and I finally decided to remove the 4.0 DLL (from everywhere) and restart the machine and try again. Success! I hope this helps someone else, remove the 4.0 assembly, reboot your server, try again.
