Aside from the power data which is the primary focus I find it interesting to see how much I was drafting thereby saving energy and exactly where I left the main group to chase down breaks (shown as not drafting).
During the race I was watching the wind data and averaging about -14mph wind speed whilst in the group, anytime I strayed outside the group a bit it dropped to -5mph so I knew I was using more energy, so I used the wind reading to tell me when I was nice and sheltered again.
The other interesting data to study afterwards is my gear selection which I can see with the gear ratio data, the amount of braking, and the times when I was out of the saddle, I also get a summary of each lap with timings and average power if I use analyze route in Issac.
So iBike gives me way more than any regular power meter will give me!

Note that my Garmin speed sensor packed up 4 miles before the end due to mud on the sensor so the final few laps are not shown