Note: I'm reviewing the Xbox 360 version.
I've been playing this game a with my family (my girlfriend, my cousin, and my cousin's girlfriend) recently, so I feel in the mood to talk about it a bit now.
This game is a third person action beat 'em up in which you can play as a wide variety of Marvel super heroes, and even a few villains if you can manage to download them (they're not included in the game). There are quite a few secrets to be found, quite a few of them in places you may not even notice at first. Together, my family and I found more than we probably would have alone, but we stilled must about thirty percent of the hidden stuff to be found. It may seem like kind of an original idea, but I can make some gameplay comparisons with the Gauntlet games. Even so, the game is still unique and fun.
Overall, it's a pretty good game, with very few problems. The most irritating problem is when some kind of effect happens to you, like being stunned or frozen, and the game glitches out and won't let you get out of that phase, meaning you're stuck like that until you die. Also, if you happen to be playing alone or otherwise have computer players on your team, sometimes they're dumb enough to fall off a cliff or die some other odd way, which can be a pain if you happen to lose a top tier character, and you can't resurrect them unless you either have Ghost Rider (even then it's not guaranteed) or if you go to a S.H.I.E.L.D. outpost station thing and resurrect them there after your character has been dead for five minutes. Otherwise, any qualms I have with the game are minor, such as not being able to do certain missions properly again after beating the game (particularly the Skrull mission where you have to face Galactus), and there aren't a whole lot of unlockable characters (unless you count the downloadable characters as unlockable).
My family and I debated a bit on who we thought the best characters were in the game. I haven't played every character a whole lot, but from what I've played, here's my tier list (in order from best to worst; also note that I do not have any of the downloadable characters, so they're not listed here):
So this all essentially means that most of the unlockable characters, save Ghost Rider and Silver Surfer, are mostly useless. However, this doesn't mean that I don't like certain characters just because they're bad in this game. For instance, Dr. Strange deserved better treatment than what he received in this game. Also, I'm disappointed that we were unable to play as The Vision. Then again, he is incredibly powerful, but so is the Silver Surfer, and he's playable.
But still, even if your favorite hero sucks, the game still is well worth checking out.