Depending on what kind of phone you have you may want to consider something like Garmin XT software and, if necessary, a bluetooth GPS receiver. On my Samsung Blackjack II (built in GPS receiver) the Garmin XT software works great with voice guidance (no text-to-speach beyond roads with numbers), 3-D view, re-route, lane/interchange display, millions of POIs, and updatability. All it is lacking is a touchscreen. It's available on MicroSD for $75 and includes all of North America on a 2gb card as well as MiniSD and full-size SD adapters. The version that includes a Bluetooth GPS receiver (if your phone doesn't have one) can be had for around $100. Both versions are compatible with WM2003, WM5, WM6, Symbian, and Palm OS. I chose this solution particularly for business travel since it is one less device to have to pack/carry. It also has a walking mode and public-transit mode for urban environments. So far it has been very useful. One consideration is it drains battery pretty quickly... about 3 hours on a full charge where the phone by itself gets about 3 days standby and around 7 hours talk time.

I was considering adding an in-dash unit to my car (along with video and bluetooth) but this made much more sense all around based on price and portability.