I've read some articles where picture editors were complaining about drift being the sound recordists fault. This makes me laugh and makes me angry at the same time. If either device is likely to have a less than accurate word clock, it'll be the 5D or 7D (depending on the audio recorder of course).
I typically use a Sennheiser G2 to send a guide track to the cameras. This should - in my mind - yield better results. The audio on the camera will be better sounding coming from my system vs the on camera mic - and will be less prone to issues like wind noise - or issues with distance to the actors (say you have a really long lens, and the actors are barely audible from camera position).
I haven't seen the plural eyes program in action myself. Most of the editors I work with love it. I have posted a few projects where the app was used (I did both the location recording and post edit/mix) and I have to say that there were numerous sync issues. I had to spend time evaluating and correcting multiple scenes.
That said - I'll still take that chore over having to use audio recorded straight to camera (maybe from an EX3 or something). The audio recorders out there tend to sound much better in general, and a lot better if you're using something like a sound devices recorder.