Well, if you'd heard about it, then you'd pretty much heard the punchline. A joke isn't that funny if you already know the punchline going in.
It works best if you discover it organically by reading the story that built up to it without knowledge of where it's going. You get to see the build up, the mounting tension between Guy and the League, and Batman in general. And so when it happens, you're expecting a big fight. The cover itself hyped, "HERE IT IS! BATMAN VS. GUY GARDNER!" And then we finally get to the big moment, and...!