As a battle series, it's really all over the place. I usually value 3 things in the genre; tactics, scale, badass pages/shots. MB consistently has the scale down and usually has the badassery down, but the tactics are on a case by case basis. There are some fights that I absolutely love, but plenty are really asspully and have completely incoherent conclusions. I guess I'd say that portion averages out to above average-good.
However, when taking into account of more than just the fights, I would say it's a really solid series. The genre shift basically happened because of poor ratings, so Nisioisin decided to have a ton of fun with it and turned it into a kind of meta commentary on the genre. It gets some very intriguing 4th-wall breaking characters as well as a few super broken ones. In fact, it gets one of the best characters in manga IMO (he's been mentioned here quite a bit).
As far as genre-shifted to battle series go, I would put Medaka Box at the top, beating out Reborn, Negima, and Rosario. I hated the first 13 or so chapters too, so I would say stick with it. No promises of course, but that's how I see it. Definitely felt that it was worth it.
