Nobody recommends using SourceSafe any more, not even Microsoft. They will now offer you an (expensive) TFS licence instead. SourceSafe is just not reliable.
I wrote about it here: Visual SourceSafe on E2. It's a bit of a rant, but that's because I had to use SourceSafe for quite a while, and the memory makes me froth at the mouth a bit.
Reliablity is the big one that will bite you. But also there are features that you may appreciate in SVN or TFS:
TFS and SVN both have atomic commits of multiple files, but Sourcesafe does not - if you check in two files "at once", it's not one operation, it's the same as checking in one of the files, then checking in the other. You can get at the state in between, where one file has been checked in, but not the other.
SourceSafe does not keep history of deleted files, file moves or renames.
Contrary to initial impressions, SourceSafe does support multiple simultaneous checkouts of the same file, if you set the right options. But TFS and especially SVN are better designed for this way of working
Unlike SourceSafe, TFS and SVN both work fine against servers on the internet (TFS just OK, SVN excellently) and SVN works well offline - e.g. if you have a laptop on a plane or train and no 'net, you can still work and compare to previous revisions or even revert, since the data to do that is held locally.
As someone else pointed out, SourceSafe, like CVS, is a "dead" product. It is not being actively developed. TFS and SVN will have next versions out some time in the future.