I just went from an iPhone with the mapmyride app (no fancy ant contraptions tho), to a 705 with heartrate strap, and speed/cadence.

With the ant adapter, you'll get comparable data, but the gps is god awful on the phone compared to the garmin. also, you'll get no elevation data with the phone, other than what the map says later.

All in all, the iPhone can serve as a computer, but it's a stop gap/ad hoc solution. The garmin will do the job far better. On rides I use my phone for music, checking my route, getting directions, or making calls. All of the data acquisition is handled by the garmin.

Don't do it.