First, just a word of clarification. The Product Description for the Nikon MB-D15 Multi-Power Battery Pack advises "To use the MB-D15 with AA batteries, use the MS-D14 AA Battery Holder. When using EN-EL15 batteries to power the MB-D15, use the MS-D14EN Battery Holder". Be assured that BOTH of the battery holders referred to ARE INCLUDED in the box and DO NOT need to be ordered separately.
Beyond this, I have not tested this item to exhaustion of the EN-EL15 battery [which does have to be ordered separately] nor tried it with AA batteries so cannot yet comment on battery life or shots fired. That said, it does feel a robust piece of kit, worthy of the Nikon Brand. It is easily and quickly attached/detached from the camera and access to the battery chamber to change power sources is simple. When attached the unit and camera combine to provide a good ergonomic and balanced feel. The inclusion of a shutter release on the battery pack is really useful when working with the camera rotated to a portrait format.
At the end of the day, it's a battery pack - but it's a very good one. There's some real thought gone into its design, look and feel.