I'm a Disney fan. I've been on Toy Story Mania at Disney Hollywood Studios several times, and I picked up the Wii version of the game last night.
In a nutshell: This game does not try to be authentic to the park ride. The biggest difference is that the stages, while similar, are laid out completely differently. There are also new minigames between the stages - one is like skee-ball, one is like a hammer game (send the alien up to hit the bell), one puts a coin in a teacup and shuffles the cups around so you can try to pick the right cup, and so forth.
The game is also on the easy side. There are far fewer targets on the screen; playing it with my girlfriend last night, we easily kept shooting the targets as quickly as they appeared. There's a crosshairs on the screen to help you aim (but you can turn it off); the crosshair seemed to move sluggishly, trailing my aim by a bit. It's possible to shoot much, much more quickly than in the 'real' game, making it feel like I was practically using a machine gun on some of the levels. And, like the park ride, there are special bonuses that can be gotten by achieving specific tasks (shooting all the balloons off the clouds, for example); but unlike the park ride where these are secrets, the Wii game starts each level by specifically pointing out every bonus you can get. It kind of dumbs down the experience.
Also, the game lacks polish. There are static loading screens with no music. The font used in the high scores board looks out-of-place. Most disappointingly, it doesn't bring you straight through all the shooting levels like the park ride does; there are menus, loading screens, and minigames between. The characters' voices can be heard encouraging you through the levels, but they repeat themselves often.
On the plus side: there appear to be lots of unlockables in the game, including the ability to play some levels in 3-D with the included blue/red glasses. I tried one level like this, and it kind of worked. And the voices are really close to how they should sound, and the characters are animated really well when they appear.
The game disappointed me somewhat; it just seems like there wasn't a lot of effort put into polishing it and making it a smooth, engaging experience. it kind of feels like it might have been better off as a collection of Flash games on the web; it's not quite as fun or clever as Toy Story Mania on the iPhone. But there are lots more options in it that I haven't explored yet. It might grow on me.