I'm nearly Real Platinum God in it. All I have left is making Super Meat Boy and finding a few Secret Room Items, and that's it. I've even unlocked Godhead, that's how much I like the game.
It's not perfect. Character balances in particular stick out to me. Not just The Lost being plague on the game, not just ??? being a challenge character, but so many characters aren't worth playing as because of how much better other characters are. Judas, Isaac, Azazel, and in my opinion Eve are just way, way better than the remaining characters on the selection screen. Some bosses (The Adversary, The Bloat, Carrion Queen, Satan, The Haunt, Mask of Infamy, to a smaller degree It Lives and Isaac) are complete bullshit and are unfun to go against. Deals with the devil and angel need help, because Devil Deals are 95% of the time the stronger path to go down with a more varied, modular item pool. And there are plenty of annoying enemies (Hosts, Lumps, Wall Creepers) in the game, but with how many rooms are seeming designed to cater to making the enemies even more annoying, I think Edmund was proud of that. Patches are rather slow to come out, and what content they do release usually introduces another bug and fixes only one or two issues. And, most surprising, despite the new engine and acknowledgement of the PC crowd, the game has pretty poor modding support outside of simple reskins.
But still, I've owned the game for not even two months and I've happily put 110 hours in it, tomorrow I'm doing probably my thirteenth try to get Super Meat Boy (FUCK CONQUEST, I'VE GOTTEN CONQUEST INSTEAD OF DEATH NINE OUT OF THE TWELVE TIMES), and am waiting eagerly for the Wrath of the Lamb-tier DLC announcement that should be coming soon. And keep in mind, I'm saying this and I don't even like roguelikes. It's well worth $15.