VVVVVV is perhaps the most fun I've ever had being tortured with a game this side of Super Meat Boy.
It's a sublime DOS inspired platformer where inverting the gravity has replaced your jump button, requiring you to completely rethink how you approach a level. Platforms are setup in a way that require incredibly precise, conscious flips and movement to surpass, at times feeling almost impossibly difficult. But VVVVVV is never unfair, offering you an abundance of checkpoints and excessively precise controls that caused me to surprise myself with what I was capable of.
The nonlinear design also allows you to opt out of some of the most challenging rooms, or revisit them later when you feel better in tune with the game. Completing the game will test your mettle regardless, but the sense of accomplishment and the ever present feeling that you can possibly surpass any challenge given just one more time makes it impossible to give up and extremely rewarding when you do eventually succeed (though all the same, "Doing it the hard way" will forever haunt my nightmares).
VVVVVV's vision is so strong and consistent, that even though the experience lasts only a few hours it was so incredibly fun and well put together that I never noticed the short length as I was simply having too good a time getting my ♥♥♥ handed to me again and again. That's probably a peculiar way to recommend a game, but VVVVVV is something of a peculiar game, coming out of essentially nowhere, putting blisters on my fingers, and making me curse and rejoice over and over while what can only be described as the best music to ever enter your ear holes played in the background.
It's good, so just play it already won't you?!