Custom workflow activities are one of my favorite extension points of CRM. The power workflows gives the customers are great as they can easily change the content of outgoing emails, todos etc. With the ability of custom workflows I can connect the workflow engine to anything in the world, from credit rating checks, integrations to other systems or just adding a bit more complex logic than what the standard activites include.
One of the major drawbacks of the current version of CRM-Online (as of march 2012) in comparisson to On-premise is from my perspective that custom workflow activies cannot be deployed in CRM-Online. I asked developer evangelist at Microsoft Sweden, André Henriksson about this and his comment was that it was probably due to the fact that the workflow foundation team didn't support sandboxing of custom workflow activities. At SharePoint Exchange Forum I had the oppotunity to discuss this with SharePoint MVP Eric Shupps and he mentioned that in SharePoint there was full support for custom workflows. This made me a bit puzzled since this implied that the SharePoint team had managed to create a workaround for the lack of isolation for custom workflow activities that the CRM team hadn't.
However, in the upcoming R8, release for Dynamics CRM 2011, Microsoft have annonced (http://crmpublish.blob.core.windows.net/docs/ReleasePreviewGuide.pdf) that there will be support for custom workflow activities in CRM Online. This is great news, but as the functionality will be based on sandboxing there might still be limitations to what can be done within the sandbox, for instance on which external connections are allowed. This remains to be seen, but the news of support custom workflow activities in CRM Online is still something I have waited for a long time!
CEO, Chief Architect and co-Founder at CRM-konsulterna AB