Adds weight, but some will say that it improves the balance when using large lenses.
The biggest benefit to most people is that added griping surface and the addition of a shutter release for vertical orientation. Also, the additional battery allows for longer shooting time.
I'm not sure about this model, but with some Nikons, the addition of a grip gives you a faster frame rate.
If you have a bigger lens and the SB900 mounted the grip may add weight but it'll also add a considerable amount of extra space to hold onto it all with. Essentially cancelling out the weight.
IF you are shooting more than the occasional outing snapshots, yes.
It adds considerable weight and bulk to the body, yes. It does increase frame rate only when used with certain battery types. It does make shooting portrait mode and switching between modes natural and comfortable
Whenever I buy a new camera I always get a battery grip on order the same day. I don't know much about Nikons but if it's like the Canon models and comes with a tray to use AA's in an emergency then it's well worth it!
true, i ended up getting a grip for my 5d, and although with the big lens it turned my 12 pound camera into a 14 pound camera i like the ergonomics of it. it feels much more secure in my hands when i have expensive glass on it. the other nice thing is that its longer shoot time and like scorp says you have a tray for aa's in a pinch
It'll make your nuts swell, the girls will swoon and makes your rig look smokin' hot.
But seriously, with heavier lenses it does help with balance. Flipping to portrait orientation is almost instantaneous and doesn't give you a chicken wing flapping in the breeze. The grip increases your frame rate. Would you rather spend time at the gym or shooting pictures for an upper body workout?