I will admit I did enjoy playing this for a several hours today. I think if I didn't play with my 3 friends though I would have been much less entertained.
The skill tree is extremely bland and boring, but the characters themselves aren't bad. Similar classes(Like Deadpool/Hawkeye) have some similar skills, but can play differently. Combat is amusing, but overall fairly bland until you get several skills, which can take awhile.
I did end up fighting a few event bosses. It usually consists of spamming your attack, dodging their attack, spamming your attack, and then ressing allies who stand in the fire.
The replayability, at least for me, will probably be low. I will play a couple characters I really want to high level.
No idea about endgame, but if there isn't something more than what I've seen through the first 5 chapters, I won't be playing any of the endgame.
Overall, its fun for a free game, it isn't terrible. I might even consider paying $20 for a superhero I really want to play.