Pretty good app that basically re-defines how much data something takes. I'm not sure if this is correct, and I can't remember where I found this info, but according to [source] the Mac OS defines data storage larger than it actually is. I believed it was said to be as 1,256kb for 1mb instead of 1,000kb for 1mb. (and so forth.) This person made a terminal command that re-programs that definition, and offered it freely. (If I find that source againI will reply here.)
Now, this app isn't worth it for people who regularly keep 100G+ of free space on their HD. (assuming it is 250G). It is very useful, works on Lion (10.7.3), but is selective in how much data it saves you. (based on the file type.) For a better idea if you really need this, look this up on youtube. you may have to search "squeeze" (which is what it used to be called apparently.)
This app certainly seems like it wasn't easy to make, but overall it should offer better features for the price they charge.
Now, this DOES take plenty of CPU power (about 1-30%). (you can monitor with the free "istat" menu for dashboard.) You can tell it what folders you want it to compress and which ones not to. Or you can add an entire directory ("desktop" for example).
For an idea of how much data you can save, it depends on how much data of certain files you have. The most it will save on is apps, docs, txt, rtf, etc. The Least you will save on is videos and picture files.