Now, as you guys probably know, I've been playing the Binding of Isaac: Rebirth for a bit. I've noticed that Isaac's heart is an item of come controversy of whether it's good or not. I've not even unlocked it yet, but I've looked on Rebirth's wiki and seen its information. I've decided to list it's pros and cons, synergies, what it does, and my opinion on it so far.
what it does: spawns a heart familiar that follows Isaac and is the thing that takes damage for him. You can guard projectiles with your body. It either overlaps or is overlapped by familiars you've collected before or after it.
Pros: Makes you immune to creep/spikes, devil beggar damage, blood donation machine damage, IV bag damage, Blood rites damage, and razor blade damage. The heart is also completely immune to brimstone damage (some say it might be a bug), which means you can do things like standing right in the bloat's brimstone area (cardinal sides and below, with your heart included) and it can't do a thing to you. XD
cons: You have to protect the heart constantly, enemies (other than turrets and probably a few others) target the heart now, and it can screw up your dodging.
Any invincibility granting item. (breath of life, Holy mantle, etc, as the invincibility carries over to the heart as well.)
Blood rites (makes it completely spammable damage, as it is infinite and, because of Isaac's heart, deals no damage to you at all.)
Razor blade. (Spammable damage up; works well with Libra, as it makes it a spammable all stats up.)
Dry Baby: If you get right after getting IH (Isaac's heart) (aka not getting any other familiars after getting IH before getting Dry Baby) then it will take all the projectile damage (aside from explosion damage, and brimstone, but Isaac's heart is already immune to brimstone.). It still get's damaged by contact damage, though.
any item that deals contact damage or 'status ailments' (E coli, the Virus, Midas' touch, Aries, etc.)
Devil beggars still give you items and blood donation machines still give you money and the occasional IV bag/blood bag, which makes farming those two easier and more useful.
My opinion: Though I've not even unlocked it yet, it can be either very good, really iffy, or kind of bad. You have to get used to the new dodging strat and protecting the heart, but brimstone/spike/creep/DB/BDM immunity and a bunch of OP synergies ( ) can make this a very good item, but that's just my two cents.