(Beaten on PC, October 9, 2014. Due to the game's pseudorandom rogue-like nature, "beating" in this case means I got one of the game's many endings.)
I usually ignored anything by Edmund McMillen because I never liked Super Meat Boy. That game felt like it was missing the point of what made old-school platforming games difficult, opting for the "hard for the sake of being hard" design that many platform games oddly emulate. Maybe one day I'll actually play SMB and change my tune.
As for Isaac, it was something recommended to me since I heard it was pretty good. I started playing it on a whim one day. Then the game became a good casual time-waster as I caught up on podcasts. I got hooked, sometimes wasting anywhere from 30 minutes to a couple hours on it.
Much like any roguelike, no two games play out identically. Sometimes I've had good runs get me near the end, only to stupidly walk into an enemy, other times I died on the boss in Basement I. That actually makes it fun, since I had no idea if luck was gonna be on my side or note.
Completing more challenges and unlocking new items made Isaac go from being a punishing game to at times a complete cakewalk. When I did get to the final boss in Womb II, I was ridiculously overpowered with so many damage and fire rate items that it took little effort. But there's always that risk where even though you're pretty powerful, you are not necessarily invulnerable, and that still makes it a challenge sometimes when I scraped by some bosses with a mere half of a heart left.
The best part about Isaac is that even though I beat the game once and unlocked Judas as a playable character, the game's not over. Replaying the game over and over gives you the chance to unlock more cool stuff, and I bet it gets to a point where dying becomes rare unless you play as Cain or Judas where health is a premium.
Now I'm starting to consider grabbing Wrath of the Lamb for an even greater challenge. I know I'll be looking into The Binding of Isaac Rebirth in the near future, but for now I'm happy shooting tears into grotesque things in a never-ending quest to save the day in the regular game.
(Sorry for doubting you, Edmund and Florian Himsl. You guys are cool people who make cool games.