I bought X on release date back in October, and just 5 days ago I bought myself a copy of Y too. I gotta say, really enjoy to have both versions because I can't get myself to start over with my Pokemon X game (still have lots to do there, I have the Shiny Charm, I breed and hunt for Shinies and I train new teams ;)) and I have over 550+ hours :)
So, now I own Pokemon Y, which I am currently playing, I can use this game to start new Save games or store my precious Pokemon in from my Pokemon X cartridge :) So now I still have a game to play from the start of the adventure till the end without having to restart my Pokemon X game.
I'm also a collector of games and Pokemon stuff, so it was only natural for me to buy Pokemon Y too eventually ;)