For kicks, I took the iphone on a 13 mile ride today using iMapMyRide.

It was very accurate relative to the Garmin.

It used 60% of the phone's battery life in 13 miles.

The "Locator" services are apparently real power hogs. Plus I was in a 3G area which uses more power.

Finally, it was in my jacket pocket, so it didn't help much on gauging speed, etc.

I'll only use it as a backup to the Garmin on those rare times where I forget to charge it up...