You are old school and should catch up.
The advantage of a smartphone is its versatility; it can use a plethora of apps and maps. With most of the Garmin/Magellan and other dedicated GPS units you're stuck with their maps, which may or may not be useful. And you have other apps, like Theodolite, that are super useful.
The downside is that using that screen and computing power means you can run out of power, and at least with an iPhone it's a pain to add juice in the field. You can't just stick in fresh batteries. And I would say that the GPS reception is almost always better with the dedicated GPS units, and some even allow for antennas. Phones are compromised in that. But there are some really good GPS units that will work with your iPhone or other smartphone to get exceptional reception, sometimes at a refresh rate higher than most Garmin/Magellan units.
There's no reason to chuck your existing unit in favor of the iPhone; it's still gonna be useful. But for some activities you may like the maps becoming available online. Even all US topos aren't created equal; some are better than others. And there are proprietary maps and special-use maps and BLM maps and... well, you get the point. We've discussed many of the options here, so start searching. You'll find some useful stuff.