Dice Player is yet another video player for Android devices, but what differentiates it from many of its competitors is its support for high definition MKV files. Obviously, for MKV files to play smoothly you need a device with a pretty decent amount of horsepower. However, there's a free trial version available in the Android Market so you can test whether the formats you need to be able to play actually work on your device before you purchase the full app.
One of the reasons why Dice Player is so fast is that it uses a phone's hardware video decoding when it's available. Using hardware decoding also helps to cut down on battery usage, so you'll get longer play time between charges.
It doesn't work smoothly on every device, but when it does work Dice Player is a great option for playing back high definition MKV files.