The MB-D11 houses one EN-EL15 or the MS-D11 AA battery holder (which holds 6 AA's I believe), aswell as the battery that is loaded in the camera body.
So that's 2 batteries at a time, however, you have to remove the grip to recharge the battery housed in the camera. (which could be seen as a pain to some).
Apparently you can prioritise the order the batteries are used in,so using the battery in the grip first would make it easier to swap in/out.
I ordered my grip from eBay for just over $200 delivered to Australia, which to me, is a fair price for the ergonomic advantage and battery life increase. Although I'm yet to speak from experience, I think I can tolerate the hassle of removing the grip to charge one of the batteries.