tl;dr: You can (and should) pick up one of these things if you have a D90 or D80, and shoot vertically or need extra juice for the camera. The Nikon MB-D80/OEM model is a bit more solid, but you can't beat the Phottix's price!
Take a look at what's hanging on the neck of almost any professional photojournalist, wedding photographer, or camera guy—you'll almost always see the tank-like and bulky body of a Nikon D1/2/3/X-series or Canon 1D-series SLR. The Phottix MB-D80 takes your D90 (in my case—it also works on the D80, though) and gives it three of the tank-like bodies' functions:
- A vertical shutter release/controls: When shooting portraits, this is immensely helpful.
- Double the battery life: You can stick an extra Nikon EN-EL3e battery in the grip (bringing your total to 2 batt's!).
- Look like a serious photographer: 'Nuff said.
The battery grip performs great, it can take a beating, and the shutter release and controls feel almost as good as using the controls on the camera body itself!
Some people (myself included) are leery of purchasing a battery grip, or anything else that's more integral to your camera's operation than a lens-cleaning wipe, that's not Nikon-branded, since the thing has the potential to fry your camera if wired improperly. But allay those fears: this grip is professional grade... and a heck of a lot more value for your money than the Nikon grip.
Appearances
There's nothing quite like a battery grip to make a run-of-the-mill prosumer or consumer digital SLR look like one of the 'big boys' (Nikon D3, Canon 1D, etc.). The grip adds a bit of bulk, an optional extra battery for extended shooting, and looks to kill for.
This particular Phottix model looks right at home molded to the bottom of the D90, and the rubberized grip is pretty easy to hold. The buttons look a tiny bit plasticky, and you can tell up-close that the labels are a cheaply printed, but from a foot or two away, nobody will ever know the difference between this Phottix model and the Nikon-branded grip.
Function
The grip is perfect for allowing quick vertical snapshots, and doesn't add enough weight to make toting the D90 a chore, even with two batteries inside. It's painless to put on and take off. To put it on, you just pop off the end cap, pop off the D90's battery door (this is the hardest part, but it gets easier over time), then slide in the battery grip, tightening it with a thumb-screw that goes in the tripod socket.
The only functional problem I have with the grip is that the only point of physical/substantial contact it has with the camera body is the tripod screw, meaning that it has a tiny bit of give when you are holding the camera with one hand using the battery grip alone. It never feels like it's going to break or come off, but it simply doesn't feel rock solid, like a D3. You get what you pay for...
Finally, the MB-D80 even comes with a nice little plastic end cover for the pins that go into the camera body, so when you store the grip apart from the camera, the pins won't bend. Just make sure you don't lose the cap!
Battery Life
Using the vertical battery grip, you can stick two Nikon EN-EL3e batteries at once, effectively doubling the amount of shots you can take with the D90. One weekend, I shot an Ordination Mass, some shots in a park, a reception, then a priests' first Mass and reception the next day, totaling probably 1,500 shots, with nary a battery charge. Of course, I never use the built in flash, and I usually only chimp a few images here and there.
The grip also comes with a 6-AA battery holster, into which you can insert a bunch of AA batteries and use lieu of one or two Nikon battery packs. The AA's won't last as long (with some stock Energizers, mine lasted about 200 shots, without flash), but they sure as heck help in a tight situation (i.e. you forgot your charger on an out-of-town trip!). The holster itself isn't all that well-made (it seems like an afterthought), and mine snapped in half after the third use. I put it back together, but duct tape doesn't hold up too well in this case :-(
Summary
You can (and should) pick up one of these things on eBay or elsewhere for around $80. Just make sure you're buying from a reputable dealer (mine shipped from Hong Kong in about a week and a half).