Volume 1 encompasses a strong plot arc. Assassins who are men with sneers or whose faces look like death, Alice experiencing good character development while being "raised" by Kumo, and a real sense of dramatic tension. And aside from the whole "no one cares about a poor cute orphan schoolgirl" noir setting, no obtrusively unrealistic parts.
But suddenly starting from the second chapter of volume 2, the assassins are hot young women who have some insane or fetishistic obsession with Alice / killing, who is constantly taking clothing damage, and as of the most recent chapter might be threatened into performing some fetish acts? With the irresistible (to mangaka) cliche of (spoiler for a plot development in early volume 2)
"bully becomes friend for seriously no apparent reason" (oh, but if that girl actually does die in the crossfire, all may be forgiven).
You could compare this manga to Karakuri Circus in many ways. The early conflicts are great, (spoiler for overall tone the series has started to take, roles characters play, and predictability)
but they had the effect of setting up the protagonist as capable of turning any fight around, and now it's starting to feel like a shounen battle manga with some fanservice elements.
Still, the first 6 chapters were gold, it's just the last 3 that have been mediocre. Anyway, this manga starts out quite awesome, and is still pretty good, but it's turning more and more into Cynthia the Mission, a bit hard to take seriously. But by volume 4 it at least hasn't gone downhill enough to be canceled, so I expect the story to not fall below a certain level of quality. I'm not dropping it when the next chapter promises to be the assassin version of Nana to Kaoru anyway...