Sygic Offline across the US ?

Hello,

I'm leaving soon on a road trip across the US and I'm still not sure what GPS system I'm going to use, iPhone or "real" GPS.
I've seen many really good reviews on the Sygic app but most people use it with 4G data plans.
I won't have any as I'm from Canada, so I would only use the GPS off line.
I will of course have maps but I will still rely a lot on my GPS. I'm sure it works really well around cities ( I've tried it ) but I'm wondering how well it does in more remote places, desert, National Parks,... ? Has anyone tried it ?

Thank you !

Lohan