The project I'm working on right now uses SourceGear Vault for SCC, and I must admit, I'm not digging it. It's like using a slightly more performant version of VSS. Except that you can set it in "CVS mode" which means it doesn't do exclusive locks. It seems very difficult to deal with adding new files (if you aren't integrating with VS.NET, which we aren't) and updates seem unreasonably slow, although that may just be me. It is a pretty big project. Doing a "Show history" on the whole tree takes over a minute to return any results at all. It does do change sets, at least, so that's a big plus over VSS.
In short, I can see tha market here (for people that are comfortable with VSS, but don't want Access databases for their backend) but I'd rather be using Subversion. I guess I'm just spoiled.