I read the reviews here and I almost didn't read Doubt!! because of some of the bad comments. But I did end up picking it up because I liked the mangaka's other work so much, and I'm glad I did! Doubt!! is definitely one of my all-time favorites now.
The idea that beauty is only skin deep seems like it's all the story's about, but actually, I think the mangaka is trying to say that girls should have more confidence in themselves. Like, Ai was an ugly girl before she made herself beautiful, and the fact that she never gave up and took action instead of just wallowing in self-pity is why she's such a great heroine. I actually really like this message and that's part of what made Doubt!! very fun to read. As for Ai's relationship with So, I do think it was weird she fell in love with him so quickly even when she knew he was a player, but I don't see anything wrong with her later actions. She truly loves him a lot and will do anything to keep him, and in the very end, she was even willing to let him go so that he can be happy. Yeah, So was a player but he remained faithful to Ai the entire time, and towards the end he only chose Shuko because he felt sorry for her, not because he loved her instead of Ai or that he didn't care about Ai.
I thought the ending, although a bit sudden, wasn't unsatisfying (the mangaka said herself that the series was cancelled and she had to change her plans and end it). The last few pages of the last chapter were really cute and happy, and I think it was better than dragging the series on.
Doubt!! is a very funny manga. A lot of times while reading it I was bursting out in laughter and slapping my legs. The art is ok, I didn't like So's drawing but oh well, it didn't make the manga any less enjoyable for me. Some scenes, Ai is drawn really pretty (esp. the scenes where she gets mad and starts yelling at people... my favorite part of the manga.)
To me, Doubt!! seemed to have a pretty strong feminism vibe and Ai is definitely the most hilarious, strongest and most determined heroine ever. The other characters were great too. I just think that people should give this manga a chance: you could end up loving it like me.