There are quite a few characters but the writing is heavily focused on developing the horror. If there is any characterisation, it is often used for pushing forth the plot and for developing the tension. Still, the characterisation, light as it may be, is quite consistent 'cos the people don't start changing at a whim and they behave according to their amount of knowledge. Also, a la seinen, there's a very light touch of politics and some criticism of the government.
I liked the horror aspects and it was quite creepy seeing people explode all over the place. 😛 The tension and horror, btw, was slowly developed and there is little shock entertainment. Instead, there's a heavy emphasis on medical and scientific elements(though I'm unsure how accurate they're) and the disturbing results from the viral effects and human reactions.
As for the ending, it wasn't that great 'cos there're ways to build more suspense instead of relying too much on drama. But ah well, it was fun reading and I suspect the manga had a rushed ending.
People who prefer lots of shock value, sudden conclusions, plenty of gore, faster pacing, etc. should stay away from this title.