His only downside is that you're probably going to miss the first Devil Deal because of his low HP, and that you need to be very good at dodging.
Without the D6, though, Isaac is not worth your time.
Because he usually needs Arcade rooms to be very good, getting to the Boss Rush as Cain is not the easiest thing to do.
While he has extremely high damage, flight and lots of Black Hearts at the start, his range is very short. This makes it very risky to get near exploding enemies (Mullibooms, dark green Champions, etc.).
He has insanely good early-game with his flight and ability to one-shot pretty much anything, but he really needs good items in the late-game.
If you are experienced enough with the game, Maggy is usually not good. At all.
Like, if you pick Maggy after 10 hours of gameplay you should be ashamed.
You can get crazy combinations that wouldn't make sense on a normal character, and this means runs as Eden are truly different than any other.
...But you can also get complete and utter tat, so this is why Eden is in the not-so-exact middle of the list.
The extra life is not very useful. Because when you die, you're probably at the Sheol/Cathedral or Dark Room/Chest, and 1 Red Heart will probably not save you.
Samson. I don't like characters that require you to take damage, but Samson is probably the best one out of the two. He is massively buffed compared to the original game, but I still don't think he is really good.
Eve. She needs to take even more damage than Samson to be of any effect, and even this sums up my opinion of her pretty well.
???. This character has nothing going for it. It cannot ever obtain Red Hearts, and starts with the Poop, which is... well...
Even the infinite Negative/Polaroid effect is not that special, because I can do this already with every other character. I just need to spend all my hearts at a Devil Deal, which is going to give me good items as well as the Negative/Polaroid effect.