We are still using CVS served on a linux server. I started using it on a project in the 90s and the repository has grown from then to include the 500K+ lines of my code + 1 to 2 times that amount of other users code. The main reason not to move to subversion is that we really like the client application on windows (wincvs). Since rapidsvn is improving I am willing to give subversion a look especially because we need to move to a server that is not hosted on our network (too hard/expensive to get firewall exceptions/DMZ in our hospital) so a user can access the server remotely.
While it does have limitations, SourceSafe works reasonably well. It's less bug-ridden and much less complex than a number of the other options in the poll. The key to SourceSafe integrity is keep the data base within limits (5GB and 10 users), regular use of the ANALYZE tool, and frequent backups.