Good, but not without its limitations,
This screen protector is a good idea, mainly for people who prefer to keep their LCD screen open and use their D5100 like a DSLR with a fixed screen. The cover is a very good fit, it clips on to the top and bottom tabs of the LCD as well as the groove that runs around the edge of the screen. because it's a thick piece of plastic you won't be able to close the LCD screen while the protector is on but, like I said if you prefer your screen open all the time this doesn't affect you. There are a few issues with this protector, however:
While this helps to minimise scratches on your camera's LCD, the cover itself isn't very resistant to scratches. It's a hard cover but it's not THAT hard. If you keep your camera by your side for a day or so you will start to notice some very fine scratching appear where it rubs against your clothing. Over time this will "cloud" the cover and affect its clarity. I've lessened this by applying a cheap protective sheet on top. Counter-intuitive perhaps, but effective.
There is no spongy/rubbery-type seal around its edge, so dust and dirt WILL inevitably make its way between the cover and your LCD screen. The biggest problem with this is that the cover and your camera's LCD can rub against each other (if you press on the cover the back side easily touches your LCD). It's not a huge amount of movement but the dust trapped between the 2 surfaces will act as an abrasive and create dot scratches on your LCD screen. Fortunately for me I was using a Vikuiti screen protector before applying this cover and I didn't remove it, so the dots that have developed are on that foil, not my camera's screen.
All in all it's an okay product. It does what it's supposed to do, but in the long run it might end up causing a few more problems despite solving the ones it was made for.