It's a pretty short game, made more dissapointing given how fantastic a ride it is. Some of the levels are glorified arena bouts too which doesn't help matters either. Go for 100% mastery, it will extend playtime a tad.
I like short games, I don't have time to go through an RPG that is 30+ hours long due to graduate school study. This kind of game is perfect to have a short term fun.
I'm playing through some PS2 budget games also. The Zone of Enders is what I'm playing now and it is also a short game which I am happy with. The game is good but the gameplay gets old fast. Last week I played The Bouncer twice and finished both games in a total of about 5 hours.
Playing the game on Hard is the way to go with Army of Zin. It really isn't SO short on Hard and going for saving the innocents on that setting is a whole new game as the Zin become hell-bent on slaughtering them instead of going for you.
AoZ has a fair number of levels and bosses IMHO for a fusion of an old-school arcade platformer and a more expansive 3D game.