To free, or not to free?
That is the question. And it's really the one you need to ask before you play this.
I think the shine of the free2play model has really started to wear off, as users become wise to what that term really means. For the most part, it either means "pay2win" or possibly "pay2continueplaying". Oddly enough, Marvel Heroes means neither of those things.
While games like Lord of the Rings Online virtually forces a decision between stopping at a certain level, or actually dishing out money, and many, many games force you to dish out money to even be competitive, Marvel Heroes doesn't.
True, you may not get the hero you want when you first start playing, but there are two very reasonable way to approach this game:
1. First of all, all playstyles are represented in the heroes you can pick when you first start playing. Regardless of the specific hero you'd actually like, you can pick one that plays similarly, and then play through the entire game without spending a cent. You can even save up enough Eternity shards (one of several in-game found currencies) and buy that hero. It wouldn't take more than a few day's time, and you could do it without spending anything.
2. You could download it, pay 4-10 bucks for the exact hero you want, and enjoy a full game for that price.
Marvel Heroes isn't pay2win, as the free heroes aren't handicapped in any way, and it isn't pay2continue, as you're given the entire game for free. The question then becomes: is it worth playing?
This is obviously a question up for debate, though I find that discussion way too embroiled with the "paying for more heroes" debate, which, as I've already said, is a different discussion entirely.
That being said, I would call Marvel Heroes a very average game.
We've come to expect ridiculously high standards when it comes to games. I remember when getting a 6-8 out of 10 from a magazine/website meant that most people who were even moderately interested in the genre/context would likely give it a try. Now we act like anything lower than a 9.0 is complete garbage.
Marvel Heroes presents a very nice chunk of gameplay and fun for the price of $0. It doesn't claim or pretend to be a blockbuster, earthshattering game, even in its own genre. It also stands on its own as a solid Hack n' Slash RPG inside a very popular IP.
The graphics aren't top notch, but they're not terrible either. You can copy and paste that sentiment for animations, effects, and enemies as well.
I actually think the game excels at character variation, as each one I've played has a very unique style that feels true to their comic book self. At 21 playable characters (at launch, anyway), you're also getting more classes than any other ARPG I've ever heard of. They're not incredibly robust, but they're not thin either, and I've had fun with all but a couple of them.
The game now runs much more smoothly than it did at launch, and there's quite a bit more to do. The population is big enough that you see quite a few players running around in any zone you're in, and the MMO-ish elements don't detract from the game in any way as far as I'm concerned.
Honestly, if you like Marvel and/or ARPG games, then it's worth a play. Path of Exile is fine too, but for a free game, it's very well made.