Cornerstone has a great set of features, way better than any other SVN GUI client. So why 2 stars? Because its only useful for very small, local repositories. If you have a significant number of files, or the repository is hosted on a remote server, Cornerstone slows down to a crawl.
This is for a very simple reason: Every time ANY file in your repository is touched, Cornerstone does a COMPLETE svn refresh on your ENTIRE repository. Needless to say, if a full update takes more than a few seconds (due to size or network latency), the app becomes frequently unresponsive.
A common workflow is, do some editing in Xcode, then switch to Cornerstone to check a diff or commit. The mere act of switching Cornerstone to the frontmost application does a full svn refresh, though, causing the app to be unresponsive for a long period of time (minutes, for me). But you cant switch away and do more work, because if you switch BACK, it will do ANOTHER refresh! The developers purposefully added this behavior, and telling them about performance problems gets you what is clearly a standard form email telling you its your fault for having such a large svn repository! Meanwhile the command line or other GUI clients (like Versions) work just fine for large repositories.
Anyway, if your svn repository is only a few dozen files hosted locally, Cornerstone will be lovely. For professional developers it is, sadly, too frustratingly slow to use.