The game is just as playable in Single player as any Dragon Quest game that came before it. Don't worry about not being able to play multiplayer if you're a loner like me. The multiplayer part of it isn't even that well-integrated, as the only progress a 'guest' character retains in the game is the stuff directly related to their character (money, XP, items). Guest characters do not progress their 'stories' even if they are participating in the story events of the host, which can be a little frustrating if you're the type who wants to play through the whole thing with someone like it was Diablo or an MMO.

Also, 5 was the best.