Apparently the game wasn't very good in its early stages, but has improved a ton since launch with a reboot.
Here's what SAs FAQ has to say on the subject.
Marvel Heroes, at launch, was crippled by a number of poor design decisions, lousy UI, and sketchy Free-to-Play mechanics. Fast forward a few months to version 2.0 and Gazillion has delivered one of the biggest turnarounds in the industry. Players coming back after having just played the beta and 1.0 can't believe it's even the same game. Significant changes since launch, include: adding ways to earn the hero you want for free by just playing the game, a huge revamp of the defense system, tons of new types of equippable gear and new gear slots, a new PvP mode, queues for co-op content, and of course a ton of new characters. Best of all, new content is being released monthly, if not faster.
If you played the game at launch and decided it wasn't your thing, give it a try! You'll find it's basically a whole new game now.
So, I started playing this recently after my supervisor at id suggested it as a game. I had noticed one of my friends playing it, but thought it was another DC Universe Online style game, but for Marvel and didn't care to try it. However, I tried it after he told me it plays like Diablo and now I'm hooked on the damn thing. Have already got 3 of my IRL friends playing it with me now too, and would like to play with some forumites.
Heroes are unlocked League style. You can either pay the real monies or collect "Eternity Splinters" to buy them. You get 200 for free when you complete the main story mode for the first time on an account, enough for either Taskmaster or a random hero. Costumes cost real money, but if you're lucky can be world drops along with heroes. The biggest downside is the game really wants you to spend real money, because extra stash space is only available by real money and unless you just don't pick up crafting mats at all, you'll probably run out of stash space quick.
I've been binging on this game recently. Already have 6 heroes in my roster: Taskmaster, Scarlet Witch, Daredevil, Storm, Squirrel Girl, and Star Lord. I think I like Tasky a lot due to his diversity, but I also enjoy Squirrel Girl for her nonsense. Storm has great aoe powers, but she's kind of lacking when it comes down to burning down bosses. Not much to say about the others except that Star Lord is a late bloomer, Daredevil feels kind of squishy, and I haven't played much of Witch yet.
I also agree with Dogma about the stashes. Though, it's currently a non-issue with me since I donate almost everything I pick up to Ant Man.