This game came out against Marvel's "Ultimate Alliance", and I heard it wasn't as good. I now have both games, and while it's not the same animal as M:UA, it's a great game in it's own way. As others have noted, it's probably closer to the X-Men: Legends games. But historically, games featuring Batman and Superman have really sucked. This is probably the best game featuring the DC Characters EVER: Bats, Supes, Wonder Woman, Martian Manhunter, John Stewart Green Lantern (Hal Jordan and Kyle Rayner GL's also unlockable), Flash, and Zatanna (who quickly became my favorite) are the main playable characters. Green Arrow and others are able to be bought as you play through the game. This may the game's weakest point, that you only get to play with 2 characters at once, and only have character selection at certain points in the game.
The game's other strong points: graphics (the cut scenes look weird at first but actually get even more awesome as you progress), sound, music, characterization and story (by comic/JLA cartoon writer Dwayne McDuffie) are also almost perfect. The game also has replay value--as you progress you power up your characters more and more, and can replay through the game with your powers on a different skill level.
In short, JL:H has been overshadowed by other games, but is highly Recommended.