I've tried quite a few of them and Versions was the clear winner in my case. Here's a summary of what I tried.
I didn't like the XCode SVN integration. I tried out all the free SVN clients I could with no luck.
I finally decided to buy SmartSVN. That didn't really work out so well. It was slow. It wasn't terribly stable and was forever interrupting sleep and hibernate. Just generally doing wonky stuff. If it had been a free OS product I'd recommend at least trying it, but I think for $50 buck or whatever, no way.
I decided to give the SVN Integration in XCode another try. I got it to work, but I was constantly having to work around it, fix problems and generally waste time. I don't have a complicated setup by any means. One developer, me, a few projects and two computers. It seemed whenever I swapped computers and tried to get them sync'd up there was some issue or another.
Finally out of desperation, I plunked down $60 for Versions and it is soooo worth it to me. It's stable, fast, user-friendly (as long as you understand SVN concepts) and, well it just plain works. I've already done a bunch of project rearranging that I was putting off b/c I didn't trust any of the other clients I had used, and it came through like champ.