Just seeing if there's any other suggestions, as I was about to ask the same question. In my case, I'm perfectly happy using non-free apps and probably prefer non-free.
I'd first be interested in just a hierarchical view of what folders contain identical contents, and then which individual files are the same. I don't trust file name and size comparisons, I want to know the contents are the same.
Gemini 2 makes me a bit nervous-- I don't know what criteria it is using to find matches, and I don't want anything making selection decisions for me. I'm also not thrilled about things digging through my iTunes and Aperture libraries.
I've also got Duplicate Detective installed, that I'll be trying. Any experience with that?