It's easy to see why this is so highly rated in Japan, and here on mangaupdates -- Kanau is a cute, believable character, and the story, while somewhat fantastic, is quite appealing. Superficially, the idea of an otaku-type suddenly having a daughter forced upon him is funny and entertaining, but more importantly, underneath that pop-culture front is a great story that explores the meaning of family, parenthood, responsibility and love. Though it's light-hearted, and sometimes relies on cheap dramatic cliches or comedy, the characters really shine, and I've been eagerly awaiting each new chapter release (19, as of this writing).
OnM does require some prior understanding of Japanese culture pertaining to the otaku lifestyle. Although most of what you'd need to understand for the story to make sense can be inferred via dialogue, some of the comedy will probably benefit from an understanding of the subculture.
Reminds me a bit of Chokkoto Sister, though I think that while the "cute" factor is about the same in both, OnM has better characters.
Well worth your time. Definitely read this.