There's a lot going on in this manga. Plenty of hilarious comedy (if often scatological in nature), for starters, and loads and loads of kinky, sexy, dirty moments, that might however turn off some people.
And yet this is only the surface. The gold here is in the relationship between the main "couple" - best described as a burgeoning master-servant par - and in the close view we get of main character Hideo's psychology. I admit I'm not familiar with this sort of worldview, but that makes it all the more fascinating to me. I wouldn't know if Hideo's progress is realistic, but it feels like it could be. At the very least, I approve of his evolution from his beginning as "wimpy, spineless kid" so often seen as the main character in manga.
Kurumi is more of a cipher though, as we see her mostly from Hideo's POW. We are never sure of the reasons behind her actions, and we don't really know what her true feelings are. We don't know her past except through precious clues strewn about the story, and although we can infer that things are probably going to end in tears, we don't know for sure, and we don't know why. In this sense, the author has cunningly put us in a place not unlike Hideo's; we blindly follow Kurumi/Sundome, never knowing where she will take us next, savouring the moments we get to enjoy together, but never getting all we want, and always fearing the end we know is coming.
I'd like to mention the gratifying surprise that was Kyouko's character growth. See, aside from the main couple, everybody else in this story is there just to slightly nudge the plot along, or, more likely, to create comic situations. As such, they don't really have complex personalities and they can be described in one-sentences, for example "scaredy doll-loving nerd". But Kyouko, who starts out like that ("dumb, popular slut"), is rather startlingly shown to have a fragile heart underneath that outwards appearance. So I'm cheering for her now.
The art deserves a note: it's really simplistic for most of the characters and for most situations, but it's really good at other times, such in every ecchi or moody scene. So it's really an aesthetic choice, and not lack of skill on the artist's part.
In closing, Sundome definitely isn't for everyone, but there's a lot of to love in this story, so I'll go ahead and recommend it to anyone old enough to be reading it.