I really like What Size - it's also on the Mac App store, but has many features that won't be allowed on the store (like checking sizes as the administrator, and actually deleting / thinning files to remove code that can't possibly be run like PPC code on an intel mac).

It is paid software - but well worth the expense for me. I have used it over slow network connections, many external drives and my boot volume to see quickly where all the space is so I can know what is possible to clean up and when it's just not possible due to there being no huge folders or files that I can afford to remove.

The find duplicates function is great and there are many other views, but it's geared to giving you the biggest space users on top no matter what view you choose.