I received my MB-D100 today. The two reasons I bought it were so I could use AA batteries when necessary, and for the vertical shutter release.
While it looks like this will meet my intended need, I am not impressed with the construction. The entire unit feels like it's made of plastic, and I don't get a feeling of durability from it. The battery compartment door, in particular, seems especially vulnerable to damage. Maybe after using an F5 and D1 I'm jaded. I expected something a little better.
For anyone interested in the details, it does allow for clever storage of the D100's battery cover door and electrical interface cover. If you store these items as provided for you'll never lose them.
The unit comes with a tray for 6 AA batteries that occupies the bottom of the grip if you don't have one (or two) of the EN-EL3 batteries installed.
The unit allows for the use of one or two Li-ion batteries as mentioned above. I assume the batteries must be connected in parallel, as the Nikon documentation states it holds "Or two Li-ion rechargeable batteries for extended shooting capability". Based on my experience to date with this camera, installing two batteries should allow one to shoot an enormous number of pictures. The largest number of shots I've put on one charge so far is over 500, and the battery was still showing full charge.
The voice memo recorder works as advertised. I was quite surprised to see that the longest recording you can make, 20 seconds, only took up about 220 Kb. Voice memos are written to the storage device as .wav files, with a file name of "SND_" and the same sequential number as the shot it's associated with. The fidelity of the sound was surprisingly good when played back on my PC speakers. It was clear over the tiny speaker on the MB-D100.
There is also a typical 10-pin remote socket on the left side, and the tripod mounting hole is centered under the axis of the lens.
The AF/AE lock button can also be used in conjunction with the sub-command dial to move the active AF area, a clever trick.
In general this is seems to do everthing billed. I just think it could have been built sturdier.