Bah, I always miss the fun.
BoI is definitely up there among my favorites as well. I've definitely got more hours logged in it than any other game in steam, and more than most in my life, the four exceptions being Starcraft: BW, Diablo II, Breath of Fire III and Monster Rancher 2.
I've completed all of the challenges, and find myself only about 6 achievements away from Platinum God, but I just can't manage to get through the final levels without taking damage. Especially Sheol. Man, there's no way I could beat Satan without taking a hit. Isaac and dead!Isaac I could potentially manage, but definitely not Satan