Eve's Mascara

Effects

2x damage multiplier.
-0.5 shot speed.
(Removed in Repentance) 0.5x tears / 2x tear delay multiplier. (The player's tear value will always be halved.)
(Added in Repentance) 0.66x tears / 1.5x tear delay multiplier.

(Removed in Repentance) Eve's Mascara is DPS neutral, though often the reduction of shot speed is a downgrade.
Combos very well with any item that scales with damage but not with fire rate, such as Rotten Baby or (Added in Afterbirth) Fate's Reward.
(Added in Repentance) Eve's Mascara is effectively a 33% increase in DPS, and the shot speed down is no longer as detrimental due to changes in how range works.

Epic Fetus: Missiles are not affected by low shot speed and the fire rate difference is negligible, granting double damage with nearly no downside.
Mom's Knife: The Knife is not affected by shot speed. Tears down and 2x damage multiplier still apply.
(Added in Afterbirth †) Trisagion: Beams turn black and do double damage. The low shot speed makes beams damage enemies more frequently.
(Added in Afterbirth) Tech X: Tech X actually benefits from low shot speed due to it staying on enemies longer, and is also not effected by range giving pretty much no downside.

(Added in Afterbirth) Diplopia: The damage and tear multipliers do not stack. The shot speed decrease does stack.
Dr. Fetus / Ipecac: Shots travel a very short distance, so having (Added in Afterbirth) Host Hat or Pyromaniac is strongly recommended.
Eve's Mascara: No further effect when stacked.
The Ludovico Technique: Not recommended. Tear travels very slowly, making it hard to kill mobile enemies.