Summary: As well as providing extra battery power, the battery grip provides additional controls including a shutter release button which is very comfortable when using the camera in "portrait" mode.
Pros: Additional Controls, Easy to Install, Fast Recharge
Summary: The first battery grip I bought was for my Nikon D5000. The main reason being that I was using a GPS accessory which takes a lot of extra battery power and having more batteries on board seemed like a good idea. In practice I found the battery drain from the GPS was not too bad but also found the grip made the camera a lot easier to use so soon after buying the D7000 I decided that a grip was a must have purchase.