I have completed 4 rides on Zwift Island now. I am using a Kurt Kinetic trainer and Zwift Virtual Power.
On my first three rides, I kept feeling that the power numbers (and speed) being show were unrealistic. I do not have a power meter, I have not had my power tested. I am basing my feelings on my effort while riding outdoors at comparative speeds and on readings from Trainer Road.
For my last ride, I setup one PC to use Zwift, another PC to user Trainer Road, and also recorded the ride on my Garmin 920XT. I then imported all three rides into SportTracks. The Zwift and Trainer Road fit files contain the power tracks, and I use the ST plugin Trainer Power to calculate the power for the Garmin fit file.
I then compared the power tracks for all three. The Zwift power track is consistently higher than the other two. It also seems that the higher the power, the larger the difference between the numbers.
FYI, the Heart Rate and Cadence charts tracked very closely.
Before each trainer ride, I make sure my tire pressure is the same, and I perform my own calibration for the wheel against the trainer to try ensure I get consistent results.
I love the concept of Zwift, I think it has great potential!