One of the first things a person who is PRO-Seiya will say to a Usagi and Mamoru fan is that Mamoru loves Serenity not Usagi. And I wanna say "Bullshit" to that! Anyone who gives you that argument OBVIOUSLY has never read the manga because in the manga, as Naoko originally wrote it, Usagi and Mamoru fall in love with each other before they ever find out about their past lives. Naoko herself said she didn't like the way the animators/writers changed Sailor Moon but even the anime can be redeemed.
For instance, in the anime, when Mamoru/Darien throws the rose at Beryl's heart and she begins to die, she says to Mamoru, "Your love for Sailor Moon is killing me." He then lays on Usagi's lap dying and looks at her face and tells her "Sorry I made you wait Tsukino Usagi with the odango atama." He doesn't call her Serenity here. It is Usagi he is speaking to.
Also, in the Sailor Moon R movie, at the beginning when they are finally together, the couple is out on the lake and when they go up on the dock, Usagi trips and Mamoru laughs and hugs her saying, "You're such a klutz....but your *MY* klutz" and they kiss deeply.
You can also see it in the Lani Linai episode (Dic/North American episode where Lani Linai is an artist and she gets Usagi and Mamoru to model for her and Mamoru looks at Usagi and thinks about how pretty she is and Usagi things the same about Mamoru. All the signs are there in the beginning, showing how they grow to care for each other for who they are as Usagi and Mamoru.
The biggest proof comes when Mamoru doesn't even remember ANYTHING about loving Usagi and yet comes to care for her SOO much in the Doom Tree series. Together they go from taking care of a baby, to Mamoru holding onto Usagi's hand as they play at the lazer arcade, to Mamoru reaching out to save Usagi from the Doom Tree, wanting so badly to protect her. That is him caring for USAGI. Not Serenity.
In conclusion, with all the time that he has saved Sailor Moon's butt, all the times he has kissed her, held her close, smiled at her, asked her to marry him, with everything, the answer to "Does Mamoru Love Usagi?" is: YES YES, 100 times YES! :)