Mine took about 30 minutes to rebuild. After that I did notice that image thumbnails came up a lot faster. It was late when it finished, though, so I didn't get a chance to play a game stored on the hard drive. I'm curious to see if it makes the Fallout 3 DLC playable.

It is worth noting that it'll trash and folder information you have for videos and games, so if you had them all nicely organised before you rebuild you'll have to do it again.