I don't see how someone would talk badly about this manga. Sure, it's not perfect, not at all, but except for Bakuman none of my favs are either.
I love mangas that can make me love it wholeheartly reading it through the first time. Finding faults rereading sits just fine with me. Which is exactly the case for 9 Banme no Musashi.
You know how most of the times female chars are absolutely freaking annoying? I came to hate female chars after a while... consequently, forgot that I'm a girl as well. No, I don't think I'm a boy nor do I want to be one. It's not as if I like guys either, but it came to be that I feel insulted and disgusted by girls from mangas animes and novels. This one, was one of those rare exceptions. It's not that all mangas with female leads are bad, but most mean they turn the manga either irritating or boring. For all I care, Musashi is a female that I can admire. The one thing though... she looks too much like a guy ><, even a girl can fall for her easily... yes, after knowing she's female.
I don't really care much about the romance part. I support her, she likes who she likes, and since I like her I wish for the best of whoever she choose. But well... reread it a few times already, 2 out of 3 I skipped those parts. I personally am more interested in her missions than her love life. The third time I read part of it... skipped the rest.
The art really isn't bad at all, it's not great either though. I do think it fits the manga very well, and I do like the style. But if anyone noticed, she has her lips closed most the time. Well, it fits her personality... but you gotta make sure you don't realize it in the first place. Otherwise, like me, I get overly concerned and my attention gets directed to her lips whenever I remember.
Also, the speech bubbles are a bit plain if you pay attention to them too. I probably wouldn't notice this had I not been reading Bakuman. Kinda directed my train of thoughts to more technical parts of the mangas...
The one thing I would have loved to see more of, is what becomes of the ppl that were involved in her missions. I want a continuation, whether involved more with UB or with other people that she saved. There were a few times when those situations came up, but only a few, and only briefly.
Specifically saying, I wanna see those classmates of Shingo's turn up more... And the people she saved before too. Simple put, I love seeing their amazed faces when they learn how great she is lol~ You know, the feeling of craving for someone's background to be exposed. The most simple and direct example would be Detective Conan.