We switched to Atelier late last year, mostly to be able to use Git and GitHub. We follow a strict GitFlow workflow. Versioning is working well as long as you follow the workflow.  We release every third week (plus occasional hotfixes) using Jenkins via a test server and a staging server.

We have 7 developers working on the same code base but merge conflicts are very rare and GitHub/Git does a fine job of solving them.

Switching branches is not a problem but you need to reload and recompile your entire code base every time (we have upgraded our dev machines seriously to be able to do this quickly).

Regarding workspaces/projects: we only have one workspace but have split the application some 50 projects. ISC recommended us to keep the projects small but given the CPU power we now have, I think we would have been better off with just having one project.

I can't say Eclipse is the best editor in the word and the Atelier code completion  is nowhere near as slick as Studio but it is worth it.

Hope that helps.