As far as art goes, it's very similar in style to Bleach-- good but not quite great. I would say that the mangaka's style hasn't really changed too much.
People who have read/seen Bleach will notice very obvious parallels between Ichigo and Gamma, both of which utilize ridiculously large swords and whose bodies contain some greater, dark being holding tremendous power. Both of these darker sides are at once the protagonist and not, and you see Ichigo/Gamma noticeably suppress them. There are some other character similarities (Inoue and "Wolfgangina" [<--- that really is her name, I'm not kidding] look very similar) but they're not quite as evident.
The characters are fairly well developed, better so than in Bleach, if you only go up to volume 4 (the last volume in Zombie Powder) to be fair. Some of the characters are pretty cool, my favorite of the bunch is probably CT Smith, a charismatic gunslinger who offers some of the best humor in the series. The characters are a bit more balanced than in Bleach (where Ichigo is by far the strongest of the original team that went to Soul Society) and can take care of themselves fairly well without always having to rely on Gamma-- something I appreciate.
In terms of settings, the Bleach and Zombie Powder universes are almost polar opposites. Bleach is a supernatural Japanese universe complete with swords and spirits while Zombie Powder is a sci-fi Western (think cowboy movies w.r.t to the Western part) universe with guns and outlaws. Between the two settings, I honestly prefer the supernatural Japanese one, although I will admit it is rather trite and Zombie Powder's setting is much more original (though it has a possible cousin in the form of Trigun).
As Zombie Powder is an action manga, it would be a tremendous oversight to discuss it without mentioning a bit about the action scenes, which I think are fairly good. They're actually quite similar to Bleach-- clean art, lots of plot twists in which it'll seem one guy is coming out on top but hey, go figure, he actually isn't, a generous helping of humor both during and between action scenes to keep the mood light. Also lots of bizarre "special" attacks and villains, ranging from a crazy circus magician to a titanic fat woman. It's a bit wacky, but if you've read the Bleach manga up to the latest chapters (with the rather bizarre and gruesome Szayel vs Mayuri fight) it's nothing you haven't seen before, anyway.
In conclusion, not a bad manga but I can't help but feel something is missing. Maybe that is due to the fact that this manga never really ended, the mangaka stopped abruptly. Maybe it's because some of the mechanics in the manga are a bit too similar to Bleach. It might also be because of the really cliche premise of the story-- essentially a search for "Rings of the Dead" to construct Zombie Powder to gain immortality. I think in the end, this was a worthwhile read for me, if only for historical purposes. If you like Western settings in manga, or if you really liked Bleach a lot and would like to see a similar series, or hey if you just have some free time, maybe take a quick look at Zombie Powder and see if you like it.
