The Nav in the BRZ has a fairly clunky user interface. It works perfectly fine, but it is a bit slow and it's not as user-friendly as some of the aftermarket models out there. It also has a bunch of safety measures to prevent you from doing things while the vehicle is moving, but you end up so frustrated with it that you'll probably end up in an accident anyway
That said, there are many reasons to pick a BRZ over an FR-S, but given how easy it is to replace the head unit with a better aftermarket model, I wouldn't count the head unit with nav as one of these reasons. It's a perfectly fine OEM system, however.