The nav is hit or miss depending on the area. For some people it is absolutely terrible, for others it works extremely well.

I was going to rip it out and replace it but haven't had to yet because it works so well in my area; quick re-routs, solid directions, rarely has a mis-hit...not bad. It will get tripped up in areas where construction has happened in the past 4 or 5 years since the map data is old (2004).

That said, I don't know if I'd use it without a backup. If it ever craps out I can just use Google Maps on the iPhone to bail my lost self out. I'd recommend having a similar fallback if this is your only nav option.