[Stormblood Budget] Where all the money's gone ?

[Zojha]

[...] and the only way you'll know whether you got a dud or not is by buying the expansion.

We're in the internet age >.>

There will be dozens of reviews and play videos as soon as it becomes playable to give you more than enough info to judge by, not just by gaming magazines but also reddit, friends, this very forum, bloggers, youtube etc.
I mean, I know the idea to not buy the expansion at launch sounds crazy, but, uh... I'm just sayin'. You could cancel your sub a month in advance, occasionally google how the expansion turned out, then decide based on reviews to give it a shot or not.