I think it comes down to battery life and ruggedness. I've not seen a civilian GPSr that is significantly more accurate than an iPhone 4/4S so accuracy is not really the issue. The argument could be made that the iPhone could be more accurate in some urban environments because you have triangulation and (possibly) wifi to hone the location.
But battery life is a major issue. The iPhone will give you a 3 hours--at most--without a recharge. Most modern GPSr units will give you at least 12 solid hours of power...some significantly more that that.
Plus, the iPhone is not waterproof in any sense of the word. Nor is at all ruggedized for hiking trails and sweaty, dusty thumbs. My Garmin is great at this. Awesome, in fact. Built like an anvil.
That being said, I use my iPhone for 90% of my geocaching and mapping.
That's because my Garmin isn't ALWAYS in my pocket.
And none of the current Garmin products are loaded with my email, text messages, calendar, reminders, baseball scores, contact database, to do lists, RSS reader, bank account balances, bus schedule, currency converter, scanner, books, flight tracker, HD camera/video, truck starter and tire pressure checker, news alerts, games, YouTube, note taker, music files, alarm clock, Amazon, weather report, stock ticker, constellation map, movies, Wikipedia, podcasts, dictionary/thesaurus, Groupon, horoscope, twitter, facebook, Netflix, Craigslist, movie theater tickets, eBay, calorie counter, UPS shipment tracker, photo library, guitar tuner, calculator, language translator, and every other darn thing I need to get through a week....
