VVVVVV is a retro styled 2D platformer by Terry Cavanagh, creator of dozens of free games. You play as the fearless leader of a team of dimension exploring scientists who inadvertently crash, and are separated. The game involves exploring the strange world you've found yourself in, and reuniting your friends. VVVVVV explores one simple game mechanic: you cannot jump - instead, you reverse your own gravity at the press of a button. The game focuses on playing with this mechanic in a variety of interesting ways. The game is designed not to artificially gate your progress. In VVVVVV there are no locks, no power-ups, no switches, nothing to stop you progressing except the challenges themselves.
All this game has going for it is its retro style, especially the music which I really enjoyed.
The gameplay though is frustrating and tedious. You die over and over as you try and precisely time your key-presses to navigate a tricky section full of spikes or other nasties. There're always savepoints before the difficult sections and respawning is very fast but these are not enough to remove the frustration.
The story is poorly written but you probably wouldn't play a game like this for the story anyway.
VVVVVV is worth trying out if you're into difficult platformers. Otherwise it's not worth your time.