To begin with, it is a known issue that the Nikon MB-D80 battery has reliability consistency problems when used with the D90. I went through 3 of them and had adaptability problems with all three. With mine, the moment I connected it, it would show power, but then after one or two shots, it would show empty. I just couldn't figure this out. Some other folks have had no problems whatsoever with them - but it is a known consistency problem here.
Finally, in order to get the grip to function correctly, I discovered that if I put one or two small strips of thin cardboard - like from a garment box (spaced evenly apart) - in between the grip and the bottom of the D90, this seemed to work, ensuring that I get a steady stream of power going to the camera - the contacts are better aligned that way (with some of the grips).
But having said that, I've tried a half-dozen other 3rd party grips also (yeah, I know...I'm a sicko but my escapades wind up helping others), and the Zeikos grip is one of the better 3rd party grips on the market, meaning it will have no reliability problems and will usually work superbly on every D90 you attach it to. So yes, if you have this grip in mind, it certainly allows for more power with two batteries installed, not to mention that these grips you a vertical grip shutter release option. The ergonomics are nice also in that the added grip provides for a better balance.
Another superb grip is the Taiwanese
Own User grip (Own User is the name of the company, BTW). They've been around for around 10 years and they make a variety of very reliable grips for a wealth of DSLR's - and even for some advanced digicams. The nice thing about their grips is that you get accessories with each grip - usually a remote, or an external power source module...and more.
Gadget Infinity is one of their biggest online vendors and they are exceptionally reliable - in essence, the B&H of the far east. Gadget Infinity usually sells on eBay and they also have their own website (see below). I've purchased quite a few items from them - either through eBay or through their website.
Here's the info link to this grip.
http://www.gadgetinfinity.com/product.php?productid=17323&cat=283&page=1
As an aside, I wound up going with the Own User grip and have the Own User grips on at least 6 of my other DSLR's.
Try and stay away from the Phottix ones because they have a tremendous reliability problem. Again - if you happen to have one and it works, well, then good for you. The reliability problem with the Phottix grips is centered around the grip slowly draining your batteries, even when you're not using the camera. I've had two of these (finally just sent them both back) because after just two weeks of two batteries sitting in the grip attached to the camera (and not using the camera), they were drained after this period. So there is a slow-draining problem reliability problem with some of the Phottix models (for many camera models).
Good luck in your quest...
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