Basically, the game is ok. It's a marked improvement over the last one, but mostly gameplay-wise. The fusion attacks are pretty nice and work well against large groups of enemies (what they were made for anyway). Everything looks pretty good, but mostly the character models and cutscenes. The campaign is fairly leghty and will keep you entertained for quite a while.
The ability to play as all 4 at any given time is a big plus, not to mention, more abilities for each of the four. While the thing is useful for just smashing through enemies, the human torch requires more skill with the fireballs and stuff, Mr. Fantastic is fun to use sometimes, with those stretchy attacks, and the Invisible Woman is fun to sneak past people and use psychic attacks.
There are some bad points, which the other reviewers have pointed out. It' not worth the £30, but definately worth a look at under £20, especially if you're a fan of the movie or comics.