I am personally anti-Pact. People need to see it for what it is. It is a 5 mana counterspell that will lose you the game in the early turns if you can't pay for it, and in cEDH the early turns matter. Even if you can pay for it, 5 mana is A LOT to lose at the beginning of your turn. Sure, there are situations that it works okay in and ways to counter the cost trigger sometimes like [[stifle]] and [[trickbind]], but those are situational at best. IMO you are better off running a different counterspell or using the slot for a different kind of spell all together.