I have the Canon one. Bad news first:
The mechanical connection between it and the body is a bit sloppy, meaning that putting the camera on a tripod (attached to the grip) isn't the most rigid setup. That screw is also gear-driven and those plastic gears can be damaged if you put them on too tightly, leading to your grip being a permanent fixture. Also your thumb is a long way from the directional controller, so changing AF points means changing your hold on it.
The good news: It works, and it has an off switch if you don't want to hit the buttons accidentally. The AA cell adapter is probably only an emergency measure, but it is nice to have the option if you run out of batteries and you can't find your charger. I haven't run my camera hard enough that I felt I needed two batteries in it, but the added run time could very well be useful if you don't want to stop or if you want to keep the door shut.
I got mine on eBay for a good price so I don't feel bad about it, but I doubt I'd have paid full retail for it.