Worth downloading and playing. The best, it's free.
The entire game is fantastic, with a calm start and an awesome ending. The characters are cool, some will make you laugh and make feel bad when they die, making the story even better. Addictive gameplay, with old-school graphics and various weapons to choose. Every boss fight is challenging, from the beginning to the end.
You can spent more time on it, and go for the alternate ending. Even having a friendly AI by your side, going for it is ridiculous challenging, but there's no way to not like more challenge on this game.
If you like short games which are really good, search for, download and play it. Cave Story will be a good experience for you. Or you can wait for the upcoming release of it if you are too lazy to look for it.