I'm not completely opposed to getting it, but knowing that the game play is focused more towards how you play card combos rather that superheroes battling supervillains via superpowers (as it should be) just rubs me the wrong way. So I'm sure the game is fun in a UNO with a tacked on theme sort of way but my inner fanboy won't settle for this.
I can definitely see what you're saying about the theme not being what you want it to be. I think you have to stretch your imagination quite a bit to get the feeling that you're building up powers as a specific superhero. My thinking is that's not what they were going for. Just more of a simpler style card game.
But to call it UNO might be stretching it a bit. There are some really interesting card combos you can make, and depending which Superhero card you have will really influence which cards you want to buy. I also find the choice of whether or not to defeat the super villain a fun one to make occasionally. If there are some really good cards in the lineup you have to decide which is more valuable: the points and one really cool card ability, or maybe some cheaper cards that fit the theme of your deck better and will help you more in the long run.
I haven't yet played Legendary (though I do intend to pick it up) but from what I hear that one also doesn't quite nail the feeling of building up a superhero. I know Colby Dauch of Plaid Hat Games is working on a superhero game, and he's really big on theme so maybe that one will get that part right.