I have used virtually every Subversion client for Mac OS X: Cornerstone, Versions, ZigVersion, SvnX, the Finder plug-in, and others. Until Cornerstone, I hated them all (well, Versions was OK), and preferred to use the command line version in Terminal.
I got good enough with svn at the command line that I stopped even really looking at new clients, until Versions and Cornerstone came out, about the same time. I tried both of them out, and Cornerstone had a better history view, and a simple interface to the Ignore command, which Versions lacked at the time. (svn ignore was the only common activity I did where I could never remember the command line syntax.)
After using Cornerstone for a while, I've switched to it from the command line, full time. I know, I know, that sounds like a pretty crappy endorsement, but I was forced to get comfortable at the command line, and once I was skillful there, it took a great piece of software to get me to move off of it.
Now I wonder why it took me so long. There's nothing you can do in Cornerstone that you can't do from the command line, but it makes everything, from basic commits to ignores to reviewing history to committing only some of your changed files MUCH easier to do. My workgroup even got our web designers to start using it, it's so straightforward.
Note: there *was* a period of time when Cornerstone didn't support the new Subversion working copy format, when Subversion 1.5 came out. This lasted longer than anyone wanted it to, because the developer had other features in the works for Cornerstone 1.5, and needed to finish the whole collection of changes. The developers handled the situation about as well as one can hope, and Cornerstone works just fine with current versions of Subversion now, and I expect that the 1.5 experience was a one-time thing, and future format changes won't get held up by feature changes. (We all hope.)