No experience other than in dev environments. No problem there. Between 4 and 5 there were problems with the scheduler if my memory serves me right, but I don't think that's a problem now.

Never had any issues on my local machine but we've always separated out major versions on prod/staging environments. From CMS R2 EPiServer made an effort to ensure EPiServer works side by side so I'd be surprised if there is a problem now.

You should be OK if you install a newer version than what is already installed on the server, e.g. installing 6 after 5.

The other way around will give you problems (e.g. installing 4 after 5). EPiServer major versions are not very backwards compatible - However I think this is fixed between 5 and 6 so that you can install these versions in any order.

Thanks everyone!

In my safe virtual playground dev. environment it seems like they play smooth together (except from an upgrade error when hitting the xForm timeout (read Troubleshooting section in http://world.episerver.com/Documentation/Items/Installation-Instructions/EPiServer-CMS/Version-6/EPiServer-CMS-60/Installation-Instructions---EPiServer-CMS-6/ ). Like you said Arild, I also heard you can run 5 and 6 together, but I wasn’t quite sure.

Cheers,
Örjan