Some people think that what Apple calls "Assisted GPS" to be less than real GPS. The opposite is true, Apple uses combined Wi-Fi and 3G signals to assist GPS is areas of poor reception like in cities where the sky is obscured and multi-path abounds.
Assisted GPS works great in areas where that information is available, so on land and by using location data from other sources. When you get out of those areas, the accuracy and time to fix is significantly worse, especially when compared to an external GPS receiver.
When I've been in those situations, it's been hit or miss. The GPS receiver in the iPhone or iPad would work for a while, then go out completely with no fix at all, then show us on a fix that was within a 30 mile radius of our actual location. There were some situations where we'd never get a fix at all or only get an unreliable fix. Obviously based on signal quality or number of satellites acquired increased reliability, but those factors change on a regular basis with the built in GPS and stay more consistent with external receivers. The built in GPS receivers in those devices just isn't designed for the type of navigation that you'd want to use it for.
In other words, they work fine when you are driving down the road with cell and wifi location data. Away from all that, not so much. I've seen it be unreliable on multiple mobile devices, so it's not a singular device. I've seen it fail enough to justify spending the $125-some dollars on an external or fixed unit, YMMV.
OTOH I don't really trust the external units attached to a mobile phone either, I prefer dedicated devices. I'd consider using my phone for navigation a last resort for anything except driving down the road.