Why this is not available by default in Windows, I will never understand. Although Windows can display file sizes, folder sizes can only be viewed in the properties folder for an individual folder, until now.
This is really useful, especially when a drive is filled and the warnings appear constantly that a drive is out of space. Rather than weed through every folder and pick out large, useless files, now you can view large, useless folders and ice them. Maybe they will learn from this and make it a feature of Windows Vista.
One word of caution…
If performance is an issue, a common tip is to turn off Windows Explorer settings that calcuate file sizes. I can only assume that this will consume extra processor usage when enabled.