The art is adorable. Or rather the chara design of the twins (just look at the cover), they are such eye candy, especially if you like lolita clothes. But the art overall is good too, clean and nice. I really enjoy looking at this manga.
I find the plot quite original, there are a few unexpected twists, almost horror moments. The beginning is annoying and somewhat cliche (hating the main chara for no reason... or rather because he forgot a promise from the time they were children), but it gets better.
As for the characters... I hate Marie. She spoils the manga for me a bit. She is that kind of annoying abusive b**** that for some reason Japanese people like in manga (I could never understand this, I don't find female on male abuse cool or funny, even in manga-- and it's so overused, I can only roll my eyes). She is mean and just super annoying. I like Tomari, even though he is a psycho and can be just as mean as Marie, he doesn't go around hitting people and overall I find him an unique character-- his circumstances, his sister complex, the fact he crossdresses, etc. I think he is a cool character, while Marie is just cliche and banal-- there were many characters like her, especially in romance harem for guys.
Keiichi is your regular shounen/seinen main character. He is a nice, understanding guy (nicer and more understanding than most guys in rl, but this is how shounen protagonists are), but he is "nothing special".
I don't get why Keiichi can't tell Marie and Tomari apart when Tomari lets his hair down / Maries dresses like Tomari (she pretends she is him in one scene) -- their eye and hair colors are totally different, wtf XD (and I will add that I really like Tomari's, so cute; I'm not a fan of Marie's red hair, or red hair in general, but I think the colorings fit the characters' personalities)
Who do I recommend this manga to? To fans of otokonoko / attractive gender bender, of course. If I didn't like the theme, my rating would be much lower-- I find Tomari and Tomari x Keiichi bits the best part of this manga. I wouldn't generally recommend to yaoi fans though, since Tomari is more of a girl with a penis (well, the manga is seinen) than a guy.