There are many of these threads already.
Brief summary:
(1) You can usually get it (much) cheaper online, depending on where you live. Apple's prices are never the best for it, except possibly when buying a new Mac.
(2) It's usually worth it, or close, when you have an iPhone, a Touch, or multiple Macs. With a single Mac, it's not all that worthwhile for many people.
(3) The email and offline storage aren't really worth it; the sync capabilities pretty much are.
So... it's all up to you and your situation.