I just caught the first episode of Marvel's Avengers Assemble on iTunes for free (thanks randychico!) and I gotta say that I dug it for the most part. It's more or less an animated version of the big screen Avengers team with references to that universe which makes it feel like an adaptation of sorts. The voice acting is a mixed bag since some of it sounds really similar to the actors (Renner/Hemsworth) but some just sound so weird (Stark mostly, with Cap too). I only watched a couple of episodes of Earth's Mightiest Heroes so I can't compare them but in this one the action was relentless and stylistically satisfying and they even brought in the Thor/Hulk feud from the movie... I enjoyed it for what it was and will give the first season a shot.