This is one of those 'perfect' games, and yes, it will take over your life. When I first found it about 6 months ago, I had to apply a translation pack to get all the text and storyline in English. It played great on my Pismo Powerbook, and it plays great on my Vostro 1500 Hackint0sh.

You shouldn't play this game. Everything else will get ruined if you will.
There is no way to play it through only one time.
There is no way to stop playing it once you started. It will take over your life completely for several hours at a time.
This game is awesome. It has ware-bunnies, weapons and a toaster. Clear a day or two off your schedule.

Thought I'd share the following information since I couldn't find it readily available anywhere:

QUICK NEWBIE GUIDE:

1. You need an english translation patch to run the game. Go to the homepage and look for a link on the left. Follow the ReadMe instructions to patch the game.

2. Default Keys (and, as far as I know, there's no way to remap them):
Left & Right Arrows = move left and right
Up Arrow = look up
Down Arrow = look down, interact with person/item
Z = jump, select (in menu)
X = shoot
A & S = switch weapons
Q = Inventory
W = room information
E = map (if you have the map item)
Tab = Toggle Fullscreen
Esc = Quit menu (or pause, if you will)

3. Your "Level" is not your character's level, but your current weapon's level. Each weapon has a max level of 3, and each level makes the weapon more powerful. To increase the level of each weapon, collect the little triangles that enemies drop when you kill them (while holding that weapon). If you get damaged, your weapon may drop a level.

That's all I've figured out after playing for about 10 minutes, but it seems to cover the basics.

Its impossible to remap keys just like the windows version.
My suggestion would be to get a controller and set it to output z, x, and etc.

Cave Story is an excellent game with a really nice retro feel. Seriously, it's perhaps one of the greatest games out there (including those that AREN'T free)!

Beautiful, retro, fun, hard but not frustrating.
Plays a bit like Blaster Master and Metroid, but with interesting dialogue and characters to boot.
This comes very close to what I think of as a perfect game.

This is an extraordinary game, made even more extraordinary by the fact that it is free. A must have for anyone who likes platform/adventure games.

The secret hard ending is so amazing. Some of the best platforming I have experienced.

Everyone here is right; quit wasting your time reading my comments and download this game!

What a shockingly amazing game.
It has eaten my entire weekend.