No rush to buy the season pass. The few times I've bought one, I've waited until the first DLC was announced - after all, even if the Pass is a pre-order thing, it generally doesn't go away until the first DLC is released. (And with the latest games, the pass seems to stay around forever. Which is a bit odd from a business standpoint - if it's a bonus for pre-ordering/early adopters, you shouldn't still be able to buy it aftewards, right?)
The passes I've bought, I've gotten good value from (Borderlands 2, Bioshock Infinite....). But I'm definitely glad I didn't jump at Borderlands:Pre-Sequel's... those DLC were disappointing.
That said, I've ended up getting all the DLC for recent Bethesda games, so I can't see the Pass not being a good deal.
edit:
Yeah, the safe way to do that is to accounce your Season Pass for a specific amount of DLC (like "three major story packs" or "two map packs and three additional characters" or whatever. That's how Borderlands 2 managed, giving a good amount of DLC for the pass, but then continuing to release lots more). Saying "this Pass is good for aaaaallll the DLC" isn't as safe. Of course, to give details on your pass, you need to already know exactly what you're planning to do.