I'm a big fan of everything Hattori Mitsuru has ever made, and I personally believe this is his best work yet. Sankarea manages a delicate balance of grimness and lightheartedness that is fairly rare in most series. There is a lot more I could probably say about what makes this manga a great read, but it's hard to put into words for some reason.
Just as you start to think that things are getting to sickly sweet, something tragic happens. Or you'll be laughing one moment and then a bombshell gets dropped. The pacing is excellent and the characters are interesting to read as well.
So far, personal character development has been pretty minor. For the most part, the characters seem to be the same from the start of the manga as they are now, but that doesn't bother me too much. The plot is what has been advancing nicely, and it's kind of a treat to see characters that already feel developed react in the situations presented to them.
The only let down is Rea. She's one of the titular characters, but so far it seems the only reason she's around is for the sake of the male lead. I don't dislike her character, but I'm hoping that in future developments, she's able to be less codependent on that front.
To be fair though, given what happens to her, it's not surprising she's the way she is.