The main use of my 7D is for vacations. Since I had a battery grip for my 40D I wanted the same for my 7D since I do take a lot of pictures during a trip and before the battery grip I would drain that battery during inconvenient times. Now that doesn't happen. If you have the money it is worth it.
The Canon BG-E7 Battery Grip is an optional power pack designed specifically for the Canon EOS 7D Digital SLR Camera. It holds up to two LP-E6 battery packs or six AA Batteries offering double-length shooting time. This is especially useful when taking advantage of the cameras Movie Mode Live View Mode and other operating modes that use more power. Furthermore the BG-E7 provides full vertical shooting controls which can be independently turned on or off as the photographer needs to enhance camera handling especially for shooting vertical pictures. Available Functions: Shutter-Release Main Dial AF Point Selection AF/FE Lock Index Aperture Exposure Compensation Button Reduce Button