0.5x tears / 2x tear delay multiplier. (The player's tear value will always be halved.)
0.66x tears / 1.5x tear delay multiplier.
Notes
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 Fate's Reward.
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.
Synergies
Abyss: When Eve's Mascara is picked up instead of being converted into a locust, it grants all of Abyss's locusts the high damage up without suffering the tears down allowing Isaac to increase his damage rate substantially the more items he uses Abyss on.
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.
Trisagion: Beams turn black and do double damage. The low shot speed makes beams damage enemies more frequently.
Tech X: Tech X benefits from a lower shot speed due to it staying on enemies longer and is also not affected by range, giving little downsides.
The tears down can be a major downside as it takes longer to charge Tech X and the damage dealt by Tech X is less than Isaac's damage when it is not fully charged.
0.5x tears / 2x tear delay multiplier. (The player's tear value will always be halved.)
0.66x tears / 1.5x tear delay multiplier.
Rotten Baby or 
Fate's Reward.
Abyss: When Eve's Mascara is picked up instead of being converted into a locust, it grants all of Abyss's locusts the high damage up without suffering the tears down allowing Isaac to increase his damage rate substantially the more items he uses Abyss on.
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.
Trisagion: Beams turn black and do double damage. The low shot speed makes beams damage enemies more frequently.
Tech X: Tech X benefits from a lower shot speed due to it staying on enemies longer and is also not affected by range, giving little downsides.
Dr. Fetus / 
Ipecac: Shots travel a very short distance, so having 
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.
Eve Unlocks 
Boss Rush Unlocks 
The Binding of Isaac: Rebirth
