It won't really work in an impossibly large room, so it
The 800 doesn't do video switching, and it has fewer speakers so the surround effect is less noticeable.
The 1000/1100 have more speakers, and they switch S-, Composite, and Component video. I don't believe that it upconverts video, so you'd have to switch your TV to the same type of input that was going into the YSP (Composite/S or Component).
The 4000 adds 2 HDMI inputs and one output, and it upconverts so that any input can be sent out of the HDMI output.
The 3000 doesn't upconvert, so you have to change inputs on your TV when you switch between source pieces with different video output types (for example, if you're watching a BluRay DVD player with HDMI output, and you want to change to a cable box with Component output, you need to switch inputs on both the YSP and the TV).
As was mentioned earlier, the YSP works by bouncing sound off of walls to create the illusion of the sound sourced being behind you. This cannot work in huge Big Box stores. Our relatively small store has three areas for demonstration, of which one is very irregular, and has no back wall to speak of. The effect works surprisingly well in that area, but much better in the regular, rectangular demonstration rooms.
If you're propped up in bed with pillows behind you, you won't hear the rear channels very well even with 'real' 5.1 surround, but I am very favorably impressed by the YSPs, and our customers have all been happy with their purchases.
-kevin