Dead Space: Extraction is the most entertaining on rails shooter I've played since Elemental Gearbolt. Was really impressed. Story was well-written and enough to encourage me to play the PC game, which is also actually pretty great.
Having now played both games, I can say that Extraction finally took care of the problem that Dead Space had, in that there are weapons other than the Plasma Cutter that are actually worthwhile to use. The line gun fires faster, the flamethrower consumes ammo slower, and the pulse rifle actually does decent damage. It's pretty amazing. Weapons that were just OK before - like the contact gun - have been beefed up to be usable primary weapons since their primary flaw (the availability of ammunition) has been nearly removed.
The downside of course is that the wii does not really handle graphics very well - it seems that only Nintendo themselves are capable of getting solid graphics out of this console without introducing terrible system lag, and games like Extraction just end up looking pretty bad - blurry, jagged, with the same kind of dirty brown textures we got used to on the playstation 2. The game looked a little worse than RE4 on the gamecube. There's a lot more going on on screen here, but at the same time this IS a rail shooter - it's not like we have to render as much.
Outside of the graphics the game has some rare (but irritating) glitches and is incredibly short. The ending is cryptic and not in a good way. Having said that, while I would not purchase the game, it is definately well worth a rental.