It's going to be interesting to see how the GPS wars play out. Phone vs device who will win? So far I've found phones do everything okay, but not great.
In another thread this package was mentioned. Seems like great competition to the current batch of bike specific computers.
I picked up a Garmin 800 for $209 delivered, thanks to a heads up from a BF thread, and bought a $40 heart rate strap. I don't care about cadence but to compare the sensor would add another $60... so $309 all in. 309 (and that was a great deal) vs 148 assuming you have an iPhone 5, I think they'll sell plenty of those at that price difference.
The often mentioned issue of phones flying off of bikes doesn't bother me, there are just as many anecdotal stories of Garmins being ejected as phones. I do think the battery life is a big deal but one that will get solved quickly. My Macbook Air has ~4 hours of, real life, battery power. I understand the newest ones are up to 8 hours and the Macbook Pro's up to 12.
I've got two deal killers that keep me from using my phone. First is rain, my Garmin has been drenched with no ill effects. I wouldn't want to put my iPhone through that and the current phone rain coats look hideous and I can't imagine actually using the touch screen with one on but who knows? I'm unwilling to carry extra stuff in case of rain, no little plastic bags or rain coats. Like the battery issue I think this one will get solved.
My second deal killer, while very personal, is aesthetics. I find a phone way too big to live on a bike and the mounts I've seen just make this worse. I'm sure there are plenty of folks who don't care about this.
For me, currently Garmin has a slim lead over using a phone. If I was Garmin, and all the rest, I'd be nervous indeed.