Existing users, 
The single-player campaign is awesome! This has all the over-the-top scripting of an early Steve Ditko / Jack Kirby comic, and TOTALLY over-the-top voice acting Birdman, Space Ghost, and Super Friends style. If that won't appeal to you, then you might want to stay away. I think most people with a sense of innocence or nostalgia will get a real kick out of it, though.
The campaign missions are varied and interesting. Prestige points pile up far too slowly to recruit new heroes quickly enough to be meaningful, even if missions are completed perfectly with no civilian damage or casualties. That's a shame, because some of the non-essential recruitable heroes are really fun to work with and spending prestige to recruit is the only way to get your own custom-made hero into the game.
Tactics are all-important during most missions. Expecting to just run up to and stomp the opposition will often lead to a quick spanking and a forced restore to your last saved game state. Choosing which heroes will participate in a mission is also important, and listening carefully to clues dropped during the cutscenes can provide valuable insight as to which characters will make up the best team. Some heroes work well together, playing off each other's strengths or covering weaknesses, while others tend to get in each other's way.
Although the heroes have essentially no AI and therefore require constant micromanagement, the enemy AI is pretty good and provided a decent challenge.
Highly recommended, would be absolutely one of the greatest games I've EVER played if multiplayer included a co-op campaign mode and the player could continue to use his solo campaign characters in the "danger room" or other missions after completing the solo campaign.
Some people have complained about crashing. I had one freeze-up the first time I played which forced me to force-quit the game and start it up again, but it's been perfect otherwise. Graphics are terrific and everything played very well on my 2.5-year-old Cube.
