I think that clusters may actually be slowing down my OS (otherwise it's the space that was used up). After I re-installed my OS and apps, I noticed that *everything* was MUCH faster. I do love the idea of saving space, but not at the cost of the speed and reliability of my overall experience.
This app should run completely seamlessly in the background, with adjustable priority/speed. It also should take fewer resources while doing its job.
The places in which you'd gain the most space would be documents, pictures, and apps. Videos, archives, audio, and other extremely large files will barely get any change. (the /system and /library is also excluded).
On Average I'd say that you'd gain 10-15% more space on your hard drive. However, I would not recommend this for SSD users because this is specifically meant for a HD.
I would recommend extra storage over compressing, because if you lose that much data it's bad, and there's most likely some files on your HD that you haven't used in months. I would recommend investing in a small SSD and replacing the disc drive with it, or a 64G SanDisk class 10 SDXC card. Running the OS/apps from either is well worth an investment, especially for the new MBP or Air.
All in all, this app is really is good if one were too lazy to sort through their data or cannot afford another storage device. It's okay for students/businesses if they have no other options. If it's just out of curiosity or wanting to make the best use of your HD, it's really not worth it in the long-run.