I think this is a great product. When attached to the camera (the D300 in my case) the camera feels great, if not a little heavy. It does not come with an EN-EL3a battery so you need to buy another if you want double the battery power. The grip can be used with only one battery, either in the camera body or in the grip itself if you just want better vertical handling, but an extra battery will give you double the number of shots. Two EN-EL3a batteries will not give you additional frame rate but the included AA battery adapter does. I bought this for grip for the vertical handling and not for the speed, but I did try 8xAA batteries in the unit to see what 8 frames a second was like and the only thing I can say is, it's fast.
It does not come with the BL-3 for the EN-EL4, but at the cost of a EN-EL4 at £90+, EN-EL4 charger at £100+ (and a spare battery at another £90+) only a loony (or professional) would go for this option over the 8xAA batteries if speed is the driving factor behind the purchase (sorry if there are any loonies out there;-)). If you already have EN-EL4 batteries from the professional cameras then the BL-3 is worth the extra £15 (at the time of writing).
I don't agree that Nikon should have made space for both batteries in the grip. I do not always use the grip so in my opinion it is more convenient to have a battery remain in the camera body rather than have to take one out of the grip to use the camera and start messing with battery doors, etc, when the grip is not being used. Having a battery remain in the camera makes the grip quick to deploy, just remove the rubber contact protector on the bottom of the camera and screw it into the tripod mount and shoot.
The build quality is great and it feels solid in the hand. There are no creeks or groans from the grip while in the hand due to being made out of the same magnesium alloy (allegedly) that D300/D700 is made. The rubber on the grip looks and feels the same as nikon use on the D300.
Eventually I will probably buy a D700 and thankfully I will not have to replace the grip, in fact it was one of the features that convinced me to buy. Thanks Nikon for thinking of the users and their future aspirations.
All in all, if you want better vertical handling, the sheer speed of the 8 frames a second, or both this is a great addition to your camera system.
Hope you find this review useful.
Mark