I work at a 150+ devcount site; we use VSTS/TFS2008, a necessity given the volume of projects and developers. But ... we are also running into ceiling issues with branching/merging and need MS consultancy to repair. TFS is by far the most mature source control system, but is actually not mature enough yet for flagship enterprise usage. Undoubtedly will become so in Rosario++. If you use VSS now and are in a 10+ developer situation, I still would advise migrating to TFS. You will however need someone (+ backup) with more than just suferficial knowledge of the workings of merging and branching. Tips and tricks are required, e.g. when it comes to issues due to renames, deletions, moves, simultaneously in changesets, branching for overlapping release management, component version integration issues, etc. etc.
Regards, Arjan Keene