Until I bought my Treg on Memorial Day weekend, I was one of those folks who never had NAV and never thought I needed it, or wanted it for that matter. Afterall, whenever I was planning a trip, all I had to do was download a map from the internet and I was good to go.
After noticing that all my work friends had NAV systems, and probably because of the peer pressure, I decided to go with NAV in the Treg and couldn't be happier. While the names of streets aren't on the maps, the street you're on is displayed in the MFI screen, all streets (without names) are on the main screen and the NAV voice tells you when and where to turn.
On one of my first drives, my wife and I took a 50 mile drive looking at rural real estate parcels in Northern California. We were taking left and right turns on a whim, and after about an hour, I new more or less where we were, but wasn't keeping track of exactly where we were. To get home, I set the NAV to my home address and followed the directions. We made it home without error and even located the closest Starbucks for a stop along the way. Yes, I would have made it home without, but it definitely made the experience more fun and I would have absolutely missed the Starbucks.
I'm sure many on this forum have had similar experiences with NAV, but IMHO, if the upgrade is within your budget, you should definitely have it, even if street names aren't on the main display. I have the CD changer in the back, so the NAV disc for the Western US stays in the CD slot in the dash. I don't do much cross-country driving, so will likely never (or rarely) need to swap one of the 11 disc in the set.
Hope this helps in your buying decision!