I haven't tried it yet but this isn't surprising to me at all. Every recent Apple product has had a slow burn until mainstream adoption except for the iPad which exploded out of the gate. I think the Mac might have had the slowest burn of them all without really penetrating the mainstream until the early 2000s with the Intel switch and the iPod acting as a gateway drug.
Heck the Mac is still growing long after the PC market matured and even while it's declining. That is what the end game for iPhone will be in my opinion. When the global smartphone market finally becomes saturated, the iPhone can still grow at the expense of Android from switchers (that's when being the highly differentiated product will help the most).