Hi, I'm successfully using DM on a WP 3.5 network.
The only problem I have is a hard nut to crack. Recently we had a crash (our host updated the database server and wiped our database tables in doing so!), so at that time, the domain mapping plugin wasn't working and the domains just linked to the main domain (as they are aliases) and not to the subdirectories of the network sites.
Now, users that visited the website at that time, have this wrong redirect in their browsers cache (confirmed in Firefox) or something and cannot visit the network sites as they are still redirected to the main domain.
To be clear:
http://www.subsite1.com should link to http://www.maindomain.com/subsite1 (as configured in the DM plugin)
during the downtime http://www.subsite1.com linked to http://www.maindomain.com because the DM plugin was also offline
now this redirect persists over the domain mapping to http://www.maindomain.com/subsite1
Uninstalling and reinstalling Firefox fixes the issue, but I cannot ask our users to do this just to be able to visit our websites.
Any other things I can try?