It is expensive, there's no doubt about that.
I don't know how addictive it is, It hasn't really affected me too much.
You don't really "need" the expansions, I'm playing with just plain World of Warcraft, but the expansions make it way easier to actually install the game, seeing as you need the content from them regardless of if you own them or not. If you're trying to install from an older copy of the game, you'll need to spend anywhere from 8-36 hours downloading and installing.