It's block-level dedupe, not file-level. This is similar to how Data Domain (www.datadomain.com, recently acquired by EMC for $2.2B) does it. In their customer base, mostly medium to large storage operations, the average customer compression is more than 20x. For a small (one-disk) system your mileage will obviously vary, but even a compression factor of 3-4x saves a lot of bits.