[tonygrue]

I've built plugins with VSTO (for Outlook) when I was at MS, but my memory is that it’s dependent on a plugin being distributed and not available for native use embedded into the excel workbooks.

You have to install Visual Studio, compile a plugin, register the plugin, and then from VBA you can call out to that COM interface if you'd like. Even if Excel users could overcome those hurdles, you still break the collaboration flow (i.e. just sharing an excel file).

Looking a little deeper, it looks like they're starting to support Javascript for the newer cross-platform add-in system (and also VSTO), but it looks like you still have to distribute your JS add-in via a web-service as opposed to being fully integrated into Excel and XLSM files.

The bit of just being able to share the XLSM file and users using Excel with no other installs or special network access, is really the make or break for me and non-professional programmer user scenarios I've seen.