Story: I really liked the idea of the Rings of Death, and that gathering them together can do something for the gatherer{s}. However, I felt like Zombie Powder was Dragon Ball Z meets Trigun. We have the aspect of gathering many of the same item to grant a wish, and the aspects of the main character having a large bounty on his head and the setting of a futuristic western. I feel like Kubo could squeeze a little more originality in there somewhere.
Pace was a thing with me in ZP. It moved at a pretty quick pace, but I'm not sure if it was TOO quick of a pace. At some points, I feel like the story was rushed to get to the main objective, that side-plots were suddenly cut off.
The rating for this manga is going to suffer because it's incomplete. The stopping of the serialization left a LOT of lose ends that I was really looking forward to having tied up. Like the mystery behind Gamma's armored arm, and how the surgery turned out. If Kubo wasn't planning on finishing the manga, then he should've planned out the story better to fit 4 volumes.
Art: The art was nice. Very proportional for the main characters. And where people were supposed to look ridiculous, they looked ridiculous. The action scenes weren't unclear, but they weren't a moving picture, either; you could tell what was supposed to be happening. There weren't a lot of background images to admire, and the setting, as I said before, was a futuristic western. Thus, there was only really a flat landscape. But it looked like a beautiful flat landscape.
Characters: Again, the rating is going to suffer because Zombie Powder is incomplete. The characters had nice little spices to them, but I think that the story was too short to get in to some character development. You learned a little bit about Gamma and Elwood. But I would've liked to see more inner monologue from Elwood, some mysteries about Gamma resolved and what the hell was with Smith. All the lose ends and stunted plot lead to weak character development. But they were original characters, so I give them a 9 out of 10.
If you like Tite Kubo's work, then you should give this a look. It's short and sweet, but don't expect too too much out of it, because it's not finished. I think it's something you should read on the side, when you get sick of whatever long series you're reading.