It's completely worth it, especially if you buy things off the store a lot. Even if you don't, you still get automatic updates (PS3 turns on late at night do DL updates while you snooze), cloud game saves (great to have just in case you system fries), early demo access, early beta access, and free games (Elektro named a lot of them). I have saved a ton of money as a member and it has been very worth it to me.
FYI. The special offer of "buy a year sub. and get 3 extra months free" is about to end very soon.