I've got a Powertap and a Garmin500...

I use both as a training tool and have the Powertap for all rides. I use the Garmin to view real time data so I know how I'm doing at the specific point in the training ride. I'm most concerned about time and power when viewing the Garmin during a training ride, but also look at speed, distance, time, HR and cadence. When racing the Garmin is more of a recording device since I only glance down periodically...Unless I'm trying to breakaway/bridge, then I'm concerned about power so I don't blow up to quickly...

After the ride/race I download the data, quickly review it and pass it on to the coach...

For me the data is a way to track my progress and plan the next training phase...