Contra has a charm that will just never be beat.
This game is considered by many to be one of the hardest games in the NES library, although if you ask me, it's nothing compared to Battletoads, Super Mario Bros: The Lost Levels, The Legend of Zelda, or even Super Mario Bros. 3. Granted I've played this game multiple times and have gotten better every time, the problem with the difficulty is the length of the game. Now, while I did say before that this is much longer than the arcade, this game can still be beaten in a matter of 30 minutes if you're good. As a result, it's much easier to memorize everything then in, say, Mario 3, which is a much lengthier game.
However, even if you become a pro at this game and can beat it in under 20 minutes, it's still a hell of a lot of fun to play. It's just one of those gems that no matter how many times you play, it's still fun to play. It can be debated that this game is what set off the entire Contra franchise, not the original arcade, much like how it can be debated that Super Mario Bros. set off the Mario series, not Donkey Kong or Mario Bros.
There's not much to say about this game that hasn't already been said, but this game is just a masterpiece in the NES library, and chances are if you have an NES you already own this game, so I'm not saying anything new. However, if you don't own an NES, it's a very easy game to get today, because you can buy it on Nintendo's Virtual Console on the Wii, or you could even hunt down an original copy, which shouldn't run you any more than $20. It's one of those games that just has to be played before you die.