I thought that the actors were just fine in Green Lantern (the standout performances for me were Mark Strong and Peter Sarsgaard, though). It was everything else that fell absurdly flat.
But yeah, it's good to see Ryan Reynolds in a good movie. I went through of period of disliking him a lot because (a) he was "oversaturated", and (b) he was oversaturated because he was in so many craptastic movies. I'm glad he's finally in a good movie where his actual talents are able to be shown off.
Reynolds as Deadpool undoubtedly ranks among the top tier casting choices for a comic book movie; right up there with RDJ's Iron Man, Christopher Reeve's Superman, JK Simmons' J Jonah Jameson, Hugh Jackman's Wolverine, etc ...