If you bought the game on GOG, but still want to play the very newest alpha, you can, simply register an account on factorio.com , go to your account, click to upgrade, and enter the SERIAL NUMBER you can find right here in you GOG games page. My review: the game is lovely. I just got it, and played for three days straight, just stopping for food and sleep. It's been a long time since a game grabbed me like this. If you like automating and optimizing an interesting world, go for it. My tip: don't read any wiki's etc, it's much more fun to figure stuff out on your own.
