The last blog entry on their website is from March 2012...:-/
Anyway, Clusters has been working ok for years, here on OS 10.9.5. In fact, when installed, and once it has compressed the files, you don't even know it's there — I was wondering if it was still working, indeed! ;-)
To check the gain, the easier solution I found is to Get Info on a big folder and compare it to the result for a Terminal command (du -sh path_to_the_folder) — multiplied by 1024×1024×1024, if you want a fair reading, Finder considering that 1 KB = 1000 bytes).
The result for my /Applications folder gives 36GB against 28GB, a nice 8GB gain!
If you're interested in HFS+ file compression, this article highlights a few utilities: http://www.cnet.com/news/unix-commands-for-managing-hfs-compression-in-os-x/
Some links are broken, but you'll easily find them — afsctool seems what you need to use for compressing files and folders without copying them.