(Some spoilers ahead, be warned.)
Some people say the worst thing to happen to Madoka Magica was Rebellion, or the Battle Pentagram, or perhaps the fanservice in it's promo artwork- I disagree. If there was ever something truly damning in the Madoka franchise, it's Suzune Magica. It's shows that maybe, if you want good sales on your novel, maybe you should put some effort into making interesting characters.
Suzune Magica is an exercise in exploring the Magical girl assassins of the pre-law of Cycles world. Those who're aware of what becomes of Magical girls after their Soul gems corrupt fully and hunt their subordinates in an attempt
to keep the world of Witches from increasing and saving the others from what they deem the worst fate possible. The main problem with Suzune Magica is how the story fits into this concept. If this was an intro into a new world where a majority of the audience isn't familiar with it's source material, Madoka magica, this could be done well enough.
But since we know what the consequences of being a magical girl are (See: Sudden and Tragic Death), we know what Suzune's motivations are. However, the story pretends that we couldn't possibly have this knowledge, and continues to dance around these consequences as though they're a huge, incredible mystery. Since this is included in Suzune's character, as being the magical girl 'in-the-know' of everything, while everyone else is ignorant, it serves to make her seem really pretentious when interacting with anyone other than Matsuri. Even though we can see her more emotionally driven side, it doesn't make her any more likeable in the way Homura becomes more sympathetic as Madoka progresses. The only likable characters in Suzune magica are Matsuri, Arisa and Chisato. That's out of six/seven main cast members, a majority of whom aren't even antagonists- They're just not enjoyable as characters.
Similarly, the story here is absolutely terrible. If they'd stuck with the straight forward idea "Suzune is a magical girl that kills other magical girls" and attempted to make everyone likable, it'd have worked. But no, they added in a completely random antagonist magical girl because...angst? Exposition? It's never made clear what exactly Kagari was added for. She tries bringing unbelievable elements to the story, and her revenge plot is far more complex than it needs to be. She does provide good foil for Matsuri and Suzune, so I suppose she can be forgiven for existing. It doesn't make her convictions any more believable, though.
The art in Suzune is nice, though the character designs are mainly kind of dumb- mainly in Suzune and Arisa's case, the former having a heavy scent of shamelessly 'borrowing' from Black Rock Shooter. None of them are terrible, but only Matsuri and Kagari's seemed particularly interesting and magical girl-esque. In general, Suzune magica wasn't awful, but I don't think it's all the great. The story falters, either telling us what we already know or giving us what we don't need (See: Kagari). Where this could have been a simple story about the harsh reality for magical girls pre-law of cycles, Suzune tried adding in a plot that simply didn't need to be there, while also not giving it's characters the attention they needed to hold up said plot. Because of this, the manga suffers in everyway, and while it might not be an awful manga, it's definitely not nearly as compelling as any of it's Franchise's other spin-off stories.