As much as I love computers and the latest technology, I would never leave the house without a hard copy of a road atlas, local, regional, or national.
That being said, I love my portable Garmin c550 because it warns you of upcoming traffic and tells you exactly what time you'll arrive at your destination, with constant adjustments relative to your driving conditions. And like everyone else has pointed out, finding food, hotels, stores, etc makes it a cinch.
Oh, and I gave the digital voice an English accent. Best. Thing. Ever.
However, I have had my share of satellite blackouts and Wal-mart locations that didn't exist, so it's definitely not 100% reliable.
But I think everyone else has already stated the obvious. Paper maps are your lowest common denominator. Don't leave home without one.