Having read other reviews of it, I can understand why some don't really like this manga. The author, Rie Takada, spins a story which could have been resolved in one volume, into ten. Its a classic thing she tends to do in most of her mangas, I admit, and its rather easy to skip a few chapters and still not lose yourself as the majority are 'filler' chapters.
Having said that, I can't deny that I definitely enjoyed Heart, and while my brain says I shouldn't enjoy it, my 'heart' says I should.
Takada has two main characteristics in her works. One is the factor I've already mentioned - the fact she tends to string out a simple story into more than needed. However the second factor is a much brighter one - the humour. I can honestly say that I've not laughed so hard, so often, at a manga since Skip Beat.
Onto the characters, I agree with the majority, Yume (the female lead) is a bit too naive and oblivious at certain points.. and the story does fall onto those oh-so-familiar cliche arcs which make me want to hit something.. I've seen them done so many times.
Shoubu is an interesting one. I think Takada for the majority doesn't delve too deep into characters past. So even though Shoubu could and did have a rather interesting (if fraught with tragedy) past which could have been explored but wasn't which I think sounded the alarm bells.
Roko and Yuuji, the 'side couple' were entertaining. I use quotations because they weren't really a couple and again, it wasn't explored enough. For a manga with so many chapters, I'm surprised to note not much was spent on the side characters.
Looking at this review, you are probably thinking why on earth I gave it such a high mark if all I'm saying is negative. I will use this example time and time again for Takada's work. The humour is ALWAYS the saving grace of her mangas. Sure, the plot was a little brittle and inflexible at times - and the characters weren't the deepest or most interesting. But something that can make me laugh as hard and as constantly as this deserves a high rating, and so I will give it just that. I'd definitely recommend it to others on the humour alone.