Actually, Martell may be the biggest loser. They use a TON of Responses themselves, and the card stops all of them, not just opponents'.
Of course, it royally screws Maesters (the Agenda, and most links, are Resposnes…) and Siege decks. So it's kinda silver-bulletish, but still has plenty of uses if your opponents are playing something other than Maesters or Stark-murder.
One of the really fun things is that it is going to make people FAR more aware of the timing windows and when Responses can be played. For example, it's best use might be to go first in Challenges, finish all of your challenges (including Responses to them), then play the event as a Response in the framework action window where "active player" status changes to your opponent. (Oh yeah, that's an action window, which has a Response step!).
Going to have to agree here, both Martell decks been working on greatly revolve around their powerful responses. However, do believe this came out due to the high use of Maester decks and nice that it is a big hit on them. Not sure though how best will be used though, I mean how many decks don't want to trigger some responses, although can plan it around your own still could hurt yourself just as much or more if planned poorly.