by Braithwhite
I'd actually run counter to some of the other opinions, and recommend to get them in release order.(Rook City, Relics, Timelines)
My reason is because they offer a learning curve when played in release order that actually ended up helping me enjoy the game a lot more. I also agree wholeheartedly with the earlier sentiment that it allows for a deeper involvement with the story.
Learning curve:
For one thing, despite the difficulty levels of Rook City villains, most of my groups awesome survivals, longest games, and hairs-width victories come from the Chairman, Matriarch, and Plague Rat. Rook city taught us- in a way that Dawn or Voss on advanced never did- that if you don't think before you play, you can get absolutely brutalized. That things can go wrong in a heartbeat. that sometimes Legacy isn't the answer- Plague rat can make his "Galvanize" more horrific for allies than his enemies.
It demanded that we master alternate strategies that took us out of our comfort zones. We all started investigating new heroes when our favorites went down, or started looking into their cards to see if there was some way we could eke out one more turn (never mind victory!).
I learned about the power of damage redirection. Passive (and active) tanking and combined powers. We'd used things like savage mana before, but never to such great effect as with the matriarch. Or brain burn against the Chairman. Or enduring intercession vs. the tri-solvent vat.
Whatever you think about the heroes, Rook city is all about accepting the possibility that if you don't bring your A-game, things will go bad. Its the only expansion that continually hammers that point home- something that (despite the difficulty of individual villains) Relics and timelines never did. Then when you get relics and Timelines, you don't run into Apostate or Iron legacy and feel like you've been blindsided. Pummeled, yes. Caught off balance and in unfamiliar territory, no.
I'm not going to talk about the heroes, because I think that they are all awesome. I disagree with the opinion that the Rook city heroes are underpowered, but everyone has their own opinion on that, each backed by facts, figures, and anecdotes.
I'd ultimately say to get them all. But doing it in order will yield a richer experience.