*Offline access is the model Subversion is designed around. You don't have to worry about going offline, because this idea is at the root of Subversion. Subversion was designed to replace CVS, which is typically used for projects with huge geographical dispersion. Think ADO vs ADO.NET.
*Offsite access to your repository is painless. Just open a port on your firewall (if you have one) and you can send stuff to subversion over the internet without doing anything special.
*Projects load blazlingly fast when you aren't dealing with SourceSafe (the speed difference is insane).
*You don't have to worry about read only files all over your hard disk just because you added them to SourceSafe. The Subversion model doesn't do file locks at all and is very slick.
*Revision commits are atomic. This means that when you are done making your changes, you hit sync. If something conflicts half-way through, you don't get a partial commit, the whole thing succeeds or fails.
*Renames and moves don't break things. I got quite sick of seeing that “if you rename this folder, it isn't the same item anymore” messages every time I started refactoring my code. With Subversion, files are maintain their identity regardless of any name changes or moves.
*Explorer integration with TortoiseSVN is awesome. Storing non-project assets like third party DLLs or documentation in your repository is amazingly simple. Additionally, since Tortoise is directory based, not project based, you don't have to worry about stupid mistakes like forgetting to add your strong name key file or some new web app images.
*Multiple concurrent versions are painless with Subversion. If you need to fix a bug for a client which is a few revisions back, just check out that revision and fix the bug. Very easy to do and no strange side effects.
*When we switched to Subversion, we switched our web projects to local, non web projects (see this article), which is also very nice. If you don't do this, you will have to deal with that lame “cannot load project information from server” message from VS for some reason, but you probably get those right now anyway. I guess when Whidbey comes around, this will be a moot point, because MS apparently decided that those web projects were such a lame idea that they are pretty much doing stuff this way.
Want to here why some others think Subversion is 10x better than SourceSafe? Check out this link:
The only real downside with Subversion is that if you like VS.NET integration, the plug-in for this isn't quite ready for prime time yet. To tell you the truth, however, because of the differences in the source control models, you don't really need VS.NET integration anyway. It might be nice to have some little icons inside the IDE to save you an ATL-TAB when you are done for the day, but it isn't definately anywere as important to have the integration with Subversion as it is with SourceSafe.