Well worth $15 even if all you do is play it like you would a single player game. Very nice atmosphere and an interesting skill system. I wouldn't say it would be worth a sub, but since you can just buy it the one time and play it whenever, it's a good value.
One warning: unlike most MMOs, this one has a lot of puzzle type quests. For some people that's a bonus, for others they might find having to look up obscure stuff annoying.