I've heard of other people having problems, but i've never had a problem with mine. Others say that sometimes it might make you get off an exit and then get back on for some reason... But it will almost always get you to your destination.
It's pretty accurate besides that. I've never had a problem with mine, and they basically have 3 checks for the GPS
A gyro that detects turns / which direction you are facing, and acceleration etc...
A tap into your vehicles actual speed via the VSS.
And the antenna to make sure you're actually where you are.
Although some of my destinations weren't quite the quickest, i've only, at most, lost about 3 or 4 minutes because it took a different route than expected. I'm quite happy with it to be honest.
I think Kenwood has really good navigation too, and is supposed to be better than Pioneers. I can't recall where I heard this, so don't take my word for it as the truth. I can only tell you about the AVIC-N3 from my own experience, and the N2 from a relatives experience.