I first received Creatures 3 as a gift back in 1999-- that would make me thirteen years old at the time. It had been at the very top of my Christmas list after I had become completely enamored with its predecessor, Creatures 2. To this day, nearly ten years later, I am still playing this game as much if not more than ever.
Aside from being far ahead of its time and still the most advanced artificial life simulation to this day, the game itself is beautiful, complex, strange, and unpredictable. It is a little hard to grasp at first, but you aren't given any goals to accomplish, so there's no pressure to learn everything at once. In the beginning you'll probably start out raising a few cute fuzzy norns, then you might start breeding them for mutations or certain behavioral traits, or you might start breeding tough creatures to pit against each other in fights, or you might start splicing them with creatures of other species to create whatever sort of monsters you like. You're playing god in this world, and the possibilities are absolutely endless.
But what keeps me playing this game years later is the small but dedicated community that continues to develop content for the game. This game was made to be community-enhanced, and there are many free tools available to modify the game in almost any way you like. Even ten years later, people are developing smarter, more interesting creatures, challenging worlds to explore, and scripts that can completely change the way the game is played. With this nearly endless amount of additional content, the game is constantly changing and practically never becomes dull.
Creatures might not fit into any standard genre of gaming; its attempt to mimic realistic life and biology put it way above any 'virtual pet game' and almost into the realm of 'science experiment.' It is a sometimes daunting, but truly unique experience that may have you hooked for years to come.