Reviews of video games have a lot of somewhat disingenuous warnings about addictiveness. "You won't be able to put it down!" But addictiveness is in fact just about the only thing wrong with Super Crate Box-- it's a really elegant tiny game that's unfortunately tempting to play all the fun out of in one sitting.
It's ingenious, though. You face a room constantly filling up with monsters; your weapon changes randomly every time you pick up a crate, and your score is the number of crates you pick up before a monster gets you. With a good weapon, it only takes a few seconds to clear the room, at which point... well, if the next crate sucks, hopefully the one after THAT will be good. Or the one after that.
The thing to realize is that although there are new weapons and levels to unlock, this isn't a game with a constant supply of treats to give you. You should think of the weapon unlocks as an extension of the tutorial-- the weapons that are hard to use will be introduced one at a time, but still all pretty fast. After about 20 minutes, once you've got them all, it's just you and the high score board.
When Portal came out, Jerry Holkins called it "slapstick", which struck me as weird. How can slapstick happen, if you're in control of the protagonist and you aren't trying to get hurt? But slapstick is about timing, and if Portal's physics are such that you sometimes fling yourself into a wall with perfect timing, it's funny! And so it is, sometimes, in Super Crate Box when a monster falls on your head, or you open fire with a gun whose recoil pushes you into a pit, or... well, I'll just say that one of the best power-ups comes with a well-deserved apology from the designers.
Unquestionably worth a few minutes of your time, and impressively deep and well-balanced for how simple it is. It's just not quite deep enough for how much fun it is.
(I've unlocked SMFT on the first arena, but not the other two, and I haven't gotten Ambush Mode. So there may be gameplay I'm missing. The stuff I *have* seen doesn't make it seem that way, though.)