I honestly didn't really like the manga very much. This post is gonna have some spoilers in it, so if you have yet to read, there's fair warning.
If I was grading the manga, I'd give it a D. The first and last stories were not good in my opinion. The manga style of humor and Ghostbusters just don't mix for me. I'm not trying to bash the style, I'm just saying that style and Ghostbusters, for me, are like water and oil. The characters didn't really act like the characters very much at all. I think the same stories could have been used with different characters who weren't even paranormal investigators. I dunno, just some kind of generic anime thing where there was supernatural stuff going on. The last one especially bugged me because they tried to tie it into Gozer and make a play on the forces at work in Ghostbusters II, but instead of the city's anger, it's the city's vanity, and it just didn't work for me.
I thought the stories in the middle and the plot with Hardemeyer were okay. There were some weird character things that bugged me, like Winston apparently keeps machine guns in the Ecto-1? Overall, it kind of reminded me of Real Ghostbusters, which was good, but I still think the characters weren't quite captured right and that's got to be one of the most important things about Ghostbusters.
I liked the art enough in all the stories, I suppose. I am a fan of the style, but again, for Ghostbusters, they kind of don't mix. All the anime characters look young and it's just another way I felt they didn't quite capture who the GBs are.