Probably one of the best game of all times, you can never get tired of playing it. It's old school if you like shooters.
It's also a progressive gam which gets harder as you pass levels and it's got the hardest endings of all and the worst Final Boss, it's almost unstoppable, but with expertise you can handle it.
I would recommend to anyone who enjoys shooting games to give it a try you won't waste your time knowing the beginning of one of the most popular games ever.