So, is it worth it?

[Rinnhart]

Also, unless they have changed this recently, when you purchase WoW it comes with a 30 day free trial.  So your first month being free you can take that time to decide if you like it or not and if you don't, cancel the account before you have to start paying the monthly subscription.

Edit:  One warning.  The most prevelant thing i've noticed of people when they make the transition from normal user centered games to the MMO genre is the shock of the patching process.  There is a lot of information that needs to be transfered from the server to your computer before you can even begin to play.  And the patches can be extremely large so prepare yourself for a couple hours of updates along with the install time if you decide to start playing.  There is a good years worth of updates/changes between the retail version and the present incarnation of WoW.
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How much patching you have depends. I'm not sure WoW has, yet, but most MMOs periodically re-release with an updated version. They may be waiting for the expansion.