I also had a poor experience from San Diego to LA. It doesn't get much simpler than that trip, yet the nav was horrible.
Every 2 minutes the voice guidance would tell me to "bear left" to keep going straight on the 405. I eventually disabled the voice commands. BTW - the voice command volume is completely screwed up and varies from loud to soft randomly.
Without warning the nav had me exit the highway, make a couple of silly switchbacks on surface streets and then re-enter the exact same highway again. It was not only annoying, but taking that exit ramp suddenly was a dangerous maneuver. I thought perhaps the car knew something about traffic I didn't, which is why I took the suggested route.