We wanted to make an application which would make version control usable and, most-of-all, approachable to Mac users.
As John Gruber pointed out, they were brave enough to commercially release v1.0 without the requisite hype or beta Versions. And unlike many of the other Mac Subversion options, Cornerstone includes not only client functionality, but also the administrative tools, so you can perform various operations on the repository itself, not just your local working copy.
Oddly enough, many clients seem to only support a subset of even the client commands, so with many of the GUI frontends, you need to fall back to the command line to switch or relocate your working copy to a different repository, move files, etc.. Curious to see how complete Cornerstone is in this respect…
Now the question is, who will be first out of the blocks to support the new features in Subversion 1.5 — sparse checkouts, anyone?