There are things that the Dead Rising engine does very well and things that it does extremely poorly. A lot of sacrifices wound up being made to support a game that can stick a couple of thousand zombies onscreen at once, which is why all the bosses have extremely simple patterns combined with massive invulnerability windows. It's still a fun game - I'm not sure how a game that involves a homemade chainsaw polearm can be anything other than at least mildly entertaining - but any part of it that doesn't involve wholesale zombie slaughter is working against its strengths.
That's the weird thing about Dead Rising: Case West. It's got more than its share of zombies, obviously, and you'll spend a fair amount of time figuring out a thousand different ways to kill them, but they're an afterthought. You spend most of the game fighting human opponents, who're only slightly smarter and considerably more durable than the zombies, and it gets annoying fast. Starting at about the ten-minute mark, you start getting harrassed by sprays of gunfire and flash grenades from security guards hanging out on a catwalk over your head, and that's going to be a fact of life for the next two hours. If you want to screw around and kill some zombies, you have to go murder four to six humans first, and it gets exasperating fast.
Case West takes off from Dead Rising 2's Ending A, right after it fades to black. Frank West has come to Fortune City to investigate Phenotrans, the company that makes the anti-zombie drug Zombrex, and he finds Chuck right in time to save Chuck's life. Frank and Chuck escape Fortune City and head to a local Phenotrans affiliate, where Frank is hoping to find his story, and where Chuck is hoping to find evidence that'll clear his name.
You spend the entirety of Case West in the Phenotrans facility, which is a storage pen for zombies and a pharmeceutical laboratory. It's not as colorful a setting as anything else we've seen in the series so far, but on the other hand, Case West is about two hours long. By the time you're somehow bored with beating zombies to death with microscopes, organ coolers, bone saws, cattle prods, and a floor waxer, the closing credits are rolling.
If you liked Dead Rising 2, Case West is a very small extra mission tacked onto the end, featuring the return of a fan-favorite character. The co-op's as entertaining as it is in the main game, they put in a bunch of new combo weapons and a few returning favorites from the first game (such as the katana and sickle), and it elegantly sets up the inevitable Dead Rising 3.
Its major problem is that it's focusing on the wrong part of the game. Case Zero, the prequel DLC for DR2, was a two-hour block of the best part of the game, delivered for half as much money. Case West costs five more bucks, sticks you in a less interesting environment, and spends the entire time throwing you up against wave after wave of human opponents. It's not the worst part of DR2, since it isn't just one big fight with a horribly designed boss, but it's nothing you'd want to focus on. Granted, the narrative practically demands that you should be fighting a lot of humans, but that doesn't make it fun.
If you played the PS3 or PC versions of DR2, go get Ending S and call it good. That neatly wraps up Chuck's story and makes Case West largely irrelevant. For 360 owners, Case West is worth a couple of playthroughs, but it's just not as fun as the main game. The only real reason it has to exist is to please fans by bringing Frank back, and that by itself isn't enough to justify getting shot for two hours straight.