Just finished the new content, spoilers ahead:
Achievements are still bugged but once you relaunch they appear so it's not that bad.
After beating Isaac in the Cathedral you go to "the chest" and it's pretty crazy. First room had 4 locked chests which had pretty good items in them like quad shot, chemical peel. Pretty much every room is a mixture of bosses. I had 3 horsemen in one room so it got pretty crazy. Final boss is ??? which is the same fight as you've seen before if you fought him. I finished the game with the Polaroid and I got a new ending, don't know if you have to have the Polaroid or not.
Achievements are still bugged but once you relaunch they appear so it's not that bad.