For the price, you can't beat this. Initially, upon unboxing, I thought it felt a tad light. However, once you're fully loaded with two batteries and its screwed onto the camera it feels good in the hand, allowing for a good grip in both orientations.
I had the same problem others have reported with batteries. I have two EN-EL14 batteries and if i load both batteries into the grip, then connect the grip, I get an error message on the camera about unsupported battery types. However, connecting the grip first, then loading one battery at a time seems to solve this.
A word to the wise, the included sync cable works great, however the 3.5mm jack on the unit itself wears out easy. Granted, i left the cable connected most of the time, and through the normal wear and tear of transport in my bag and use it eventually stopped working. I might have avoided this if I took the time to disconnect the cable during transport and storage. Eventually, the vertical shutter only worked if i pressed on the cable connected to the grip. Finally, during a rushed shoot, I pulled a little too hard and the jack broke off inside of the unit. I'm not too disappointed due to the price and will likely purchase another unit in the near future.
An added advantage of this unit is it stabilizes the camera a bit. This is huge, as I predominantly use the D5100 for video. It allows me to shoot much more handheld then I initially did. By changing the camera's center of gravity, it's not nearly as lens heavy.
An interesting note, my unit's battery door is stamped 3100, but works just fine.