Games Workshop's Warhammer 40, 000 game (or 40k as it's more commonly known) has been a staple in the science fiction wargaming genre for two decades. Using innovative, at the time, squad control and vehicular control, it has been mimicked by other games ever since. It has evolved for being a hero-focused game to a squad-focused game during its time, but it still retains the basic feel.
The game has a plethora of different cultures and attitudes, except they are all grim and existential. There is no happy paradise culture represented and the closest one is a fascist regime based on technology. This is great if such a thing appeals to you, but if you want prouncing ponies and happy elves, this is not the game for you. That doesn't mean that it's all grim, since the armies are portrayed as being brutal pragmatists so if you consider each culture as simply fending for itself, the universe is not so harsh and grim.
The miniatures are great. There are some bad miniatures of the bunch, as happens in all games, but overall the miniatures are fantastic. They are highly detailed and most of the range is plastic, so it's very easy to convert them to your liking.