I'm a big fan of Nav systems since I get lost going around the corner plus, it's just really cool.
That said, I own a portable nav system (Garmin Nuvi 660) that is fantastic. The obvious benefit of the portable is that it can be used in any vehicle I drive. This is helpful also, when I go on vacation or travel for work. I take the nav with me and use it in my rental car. Very useful in a strange town.
From an operational standpoint, one of the things that bothers me with the built-in systems is that they are typically located too low for convenient viewing. Although they give voice directions, nav systems aren't perfect and requires in my opinion, audio and visual of the map road ahead. One of the only vehicles I've seen with a top-of-the-dash nav system is the new Acura MDX.
If your're interested in a portable nav, I highly recommend the Garmin Nuvi 660.
Rather than trying to explain all the features, here's the link.
http://www.garmin.com/products/nuvi660/
It's pricey for a portable but cheaper than the Acadia option and has more functionality.
Curious what you think.