It is just as accurate as any sand alone GPS. The Navigon app is actually better than a Nuvi and will hold all of North America. I have sold my stand alone GPS's and now only use the iPhone and iPad, I couldn't be more pleased.
I can't speak to the software comparisons between Nuvi and Navigon since I haven't used Nuvi, but the Navigon software rocks pretty good!
As for accuracy, it's a common perception that the iPad/iPhone has 'the same' accuracy as a standalone GPS...and while for 80% of navigation, this is true, there are a few areas where almost any phone/tablet will fall short of a dedicated GPS. The most common is in brush/trees/mountains - (newer) standalone GPS units use better dedicated GPS chips (SirfIV or even SirfV that can access GLONAST)...99% of cell phones use an intergrated wifi/GPS chip that can't hold a signal in rough terrain.
I've seen this regularly comparing my Garmin Montana and current Motorola Atrix (but have also tested iPhone 4 & my BB Torch). In most areas, the signal's are just about the same - it's when inside a car (not just on the dash, but like the back seat), in thick brush, or deep canyons/mountains, the Garmin is just...better...so many less lost signal warnings, etc. One area that I find the Garmin far outclasses cell phones is when it's packed close to my body (ie when hiking) - the Garmin isn't as affected by my body blocking the signal.
...and it should be noted, if you are using this exclusively in the car or elsewhere you have 'power' any phone will work for most of what you want...but if you are planning any extended time away from power having a dedicated unit that you can change batteries with makes a huge difference. My experience has been in heavily tree'd areas, or elsewhere the phone loses signal, it really burns the battery. Last time I tested phone v gps (my Garmin against my Atrix and my exe's iPhone 4), the phones were getting run down on battery after maybe 4-5 hrs, whereas the Garmin was still at least 50%...plus I had 3 AA batteries in my pocket
Don't get me wrong, phones/tablets work great as regular GPS units most of the time. I'd certainly say go for it on the tablet! If you find there are signal issues, you can always get an external dedicated GPS unit and BT it with the iPad (would probably require a jailbreak, something that's really easy). If, however, you spend more than 1/2 day away from your truck and want to use a phone/tablet, I'd caution you about battery life as it will likely become an issue.