Only recently started reading the manga, but if it's in anyway similar to the game and anime I'm sure I'll love it cover to cover.
Yes it's slow moving... but that makes it even better. I've watched the gekijouban and didn't like it because it moved too fast, it destroyed the 'rythm'.
I notice lots of people complaining (not only about the manga, but also the anime) that the main character shouldn't have taken all the routes the game has (which lead to a somewhat inconsistent story).
I disagree. Sure, they could have focused a bit more on merging the different routes to create a consistent whole, in the current state it's more episodic with small story arcs...
But I like these relaxed kind of stories with a wider perspective more, one of the things I don't like about most visual novels (and anime based on a VN's 'true route') is that you can only take one route and thus forgo loads of interesting (and necessary) character development. More often than not I'm simply not interested in the main route because it's so limiting and omits pretty much the surroundings of the protagonist.
I can't really put my finger on why I like this story so much...
It has both parts I like and don't like,
It's sad and tragic... and heartwarming,
It's complete fiction, yet depicts human nature very well and is pretty realistic,
It's shallow and simple, yet if you pay attention is deep and moving.
Sometimes it may feel what you're watching/reading is filler material... yet it is something integral to the story and probably wouldn't have worked without.
It's more than just about overcoming tragedy,
It's more than just trying to find out who the real 'you' is,
It's more than just youngsters struggling with becoming adults (coming of age),
It's more than just true friendship...
The game has been marked as a 'nakige' or in layman's terms a 'crying game'... On of the things Key games are famous for. I'm usually not easily moved by games/manga/anime/movies/etc. but this one brings me to tears every time.
So yes, it may not be for everyone... But you have been sufficiently warned, so don't complain about the basic things. Complaining that a 'slice of life' story is bad because it's slow moving is like complaining that a 'schwarzenegger' movie is bad because it has too much action.
P.S. not all the routes the game has are depicted in the anime (not sure about the manga)...