I am a convert. I admit it. The Edit-Merge-Commit model for source control has me hooked. I wish all of my projects would use source control software that fit that model. I also want to be able to do commits and updates from either the command line or from within my development environment.
So I've been using AnkhSVN for SubVersion support in Visual Studio 2008. AnkhSVN works really well, and is free, but has some limitations. Mainly, you have to set it as your default Source Control tool. That means that when you go back to work on a TFS code base, you have to remember to change your Source Control Provider, and back and forth.
VisualSVN works differently. It doesn't even show up in the Source Control Provider drop down (which made me pause for a second), but has it's own menu in the Visual Studio toolbar. So, on a whim, I opened up a TFS solution - sure enough, got prompted for my credentials, and connected just fine. While that was open, I fired up another instance of Visual Studio and opened up a SVN based solution, and it worked seamlessly.
That reason alone has me convinced that VisualSVN is worth the price of admission.