If you are splitting costs remember that this is an online game, so only one person will be able to log in and play at any given time. You'll never be able to play with each other. If you want to do that, you'll each have to buy your own copy.
As for is it worth it, over 6 million people seem to think so
It's a lot of fun though like most other games of this type the grind as you progress gets tedious and sometimes playing seems like work rather than like fun. And the game changes significantly once you reach the highest levels.
But as for what's fun about it, you get to run around and kill stuff with your friends, you get to collect equipment and grow your character over time, you get to do quests and see all kinds of different areas and things. I like exploring these games.
It costs $13-14 / month to play, plus the initial purchase of the game client.