I have mixed opinions on my "genuine Nikon" MB-D11 grip.
It does make the camera fit my hand better, and I don't have particularly large hands. Without it, my pinkie finger rests under the camera. With it, it rests on the grip. I like that better.
On the down side, for the money, I'm not impressed with the quality. I read stories here about guys with knock-off grips that sound better made than my $200+ Nikon. The rubber (which isn't an exact match to the rubber on the camera) started coming off, and (far worse), the joystick control broke, rendering the grip unusable. This was inside of a year of use, where I've taken around 25k shots. Nikon repaired it all at no charge, of course, but still. Who needs the aggravation? I had higher durability expectations than that.
For me, the single best reason to have a grip is the "oooohhhh" factor it creates with the camera. Seriously. I do a fair amount of event shooting. If you want to get the best results, you have to look the part. Show up with a garden variety DLSR, and you're just another guy taking snapshots. Show up with a gripped DSLR, and you're suddenly a PHOTOGRAPHER.
Case in point: Last month I was asked by a client to take photographs at a MADD function that he was sponsoring. I brought my gripped D7000, my 18-200, and a SB-700 flash. I had all kinds of people ask me "what newspaper do you work for" and such. While that's somewhat gratifying in and of itself, what's more important is that by presenting the image of a "REAL PHOTOGRAPHER" you will find that people are far more willing to allow you to get the shot you want. I had four different individuals hand me their business card and ask me to email them specific shots.
In a sea of people with iPhones and point-and-click cameras, you need to stand out.
Edit: One more point: I've never come anywhere near exhausting a single battery in any shooting situation. The fact that the grip allows you to have two batteries loaded means nothing to me. In fact, I never have a battery in the camera compartment at all, just the grip.