Good fun for fans of the X-Men and those new to the heroes alike.
This game was really fun. It includes lots of well known X-men characters from both sides of the alignment aisle. It has numerous boss fights so that you can fight against the likes of Grizzly, Sauron or the Living Monolith. Each Act is rather large, and the game as a whole takes enough time to keep you occupied but not so long that you're bored of it.
Some of the more fun things that are included which make the game more fleshed out and interesting are Trivia and the Danger Room. Trivia is an easy way to level up in the early acts if you know your X-men stuff pretty well (i'd say maybe half to 2/3 of it can be picked up through the acts) and the danger room is just fun to do.
Additionally, the ability to play with someone else means that you have the opportunity to play a genre with other people that you normally wouldn't, and can make it easier, since combos with the computer can be kind of hard to execute.
Having said all that, there are some qualities of the game which are less than stellar. For instance, the game isn't really all that hard. Even when you take lots of damage fast, you can spam health potions. And the game lets you carry over 30 of them, so you kind of have to try to run out.
Another problem is that the characters are not balanced. Some characters are clearly better than others, and trying to use certain characters past the beginning will just lead to frustration because anyone you would like to replace with will be too low of a level to be any help.
Lastly, I found the replayability of this to be pretty low. I went back to play it on Expert or Hard or w/e they call the highest difficulty with a different set of characters, and just found myself to be bored by the time I reached Act 2. So maybe just rent it and kill it in the course of a single weekend or so.