Well it all depends if you want to set out your stall the way im looking at it is, I will have my WoW account ragardless if i play or not for the next 9 months Signed up in Novemeber I think it was, heck I changed my bank cards missed payments never got a mail off blizzard, yet the deal is still showing active, I get to play wow when i like which is like 2-3 times a week max now but still id have to pay to do that anyways, I get Beta access which more often then not isnt enjoyable due to bugs, but i get basically to play diablo 3 for free if i dont like it Ill just cancel my subs with blizzard they can have their game back but since I dont like it (hypothetically) and as for the having to buy Mist Xpac well most of the people who play WoW will do it anyways at least this way we get to have a look at it, also you can continue subscribing even on a vanilla only account if you wish so your not forced to buy the xpacs.
So my conclusion is I pay a year which I will play sometimes usually 1-3 days a week 2 hours a session, I basically get Diablo 3 for free, I get to trial a Xpac which if i dont like the beta I wont buy saying me the cost of the Xpac. So for me it was a decent deal if you signed up when it first started, but as the weeks pass it becomes less and less value for money.