I've got the garmin 800 and I love it. I love the fact that I can record cadence, hr, speed, elevation, grade and mapping all in the same small footprint unit. Furthermore I need to be able to see my numbers while I'm rolling, and having the device crammed in my pocket just wouldn't do. I host some rides and load maps into the garmin which gives me turn by turn directions, which is nice to have when I've got others counting on me to direct them through a new course.

On my solo rides, depending on the course, I'll use the ipod function on my iphone and listen to music. Beyond that my phone is only there to be a phone. I've tried MMR and it killed my battery on a 40 mile ride. Last week a few friends and I rolled 100, which would have exhausted my iphone battery, therefore I find the iphone as my data collection device an unacceptable solution.

In a pinch the iphone and associated app will work, but for my money having a dedicated bike computer/gps is essential.
k