First and foremost, it's portable, which means it's the perfect app to throw on your PC repair kit thumb drive. Second, where some of these tools can take a while to analyze your hard drive usage, FolderSize seems to run really fast and light. On the downside, it's not as feature rich as most of the alternatives, and you can't drill down through the results or even open folders in a new Explorer window from its interface. You can, however, zoom in and out on the window for a closer look at folders using your mouse's scrollwheel.
If you're going to do some serious file analysis where you can install something, I'd suggest going with WinDirStat. If you just need something fast you can carry on your thumb drive, this app seems like a good option. FolderSize is a free, open-source download, Windows only.