Heard lot of good stuff about P90X. I guess a towel would work on a wood or tiled floor too, in place of furniture sliders.
What the Power Wheel buys you - when its strapped to your feet - over furniture sliders/paper plates/towels under your feet:
1. It's inherently less stable, so your core, particularly the obliques, have to work more to keep the wheel from tipping over to either side.
2. Pushups are more difficult because the body is already doing extra work to maintain a stable alignment
3. Some exercises are just better suited to the wheel.
4. The wheel can be used on grass, sand, carpet, etc.
5. Elastic bands can be attached to the wheel to increase resistance for some exercises (eg. Hip-Ups, Pike Ups).