I have nav and IMO, 99% of the time I don't need it, especially when driving around town. But that 1% of the time, it's worth every penny. I find it handy when I'm stuck in traffic or if there's an accident or slow down and I want a detour.
On the other hand, a portable nav is cheaper and you can take it car to car. That 1% of the time you need nav like during a commute it might not be there with you. The real question for me when getting nav was how much integration did I want. I could go the portable nav route for much less or aftermarket head unit for some savings and most likely DVD movie playback. But there wouldn't be Bluetooth phone and audio integration for portable nav (mutes audio volume); if it's not on and I need to use it, it would take at least a 20-30 seconds to lock in the satellites to find my current location. Backup camera for aftermarket would take a decent amount of work to install. For both, theft was a concern.
Cost aside, the factory nav works very well. Only thing that sucks is it locks out a lot of (programming and setting) functions when you're moving for safety and liability reasons, but there are work arounds and bypasses around the lock out. The route calculation time is amazing, very fast and it reroutes you within seconds of missing turns or going off route. The 2006+ nav has clearer junction and off-ramp markings telling you the number of lanes on the current highway and how many lanes are on the off-ramp. The previous gen nav, just has a generic picture that shows a "Y" with an arrow for the off-ramp. I also find handy is when driving back from the sierra after snowboarding in the dark and a storm and is with nav, I can anticipate which way the road curves and turns ahead of time. I set mine to display gas stations so on long drives I know how much further I can push a tank of gas before the next station.