Google Maps, which is still technically in beta (is anything at Google not in beta?), is already a fantastic tool. In fact, I've stopped using MapQuest all together. The main reasons:
Google Maps allows you to scroll around in real time
Just click anywhere on the map and drag it around. It's like having a huge map of the US on the desk in front of you.
The maps just look better
High-resolution, nicely shaded maps with wide streets (when zoomed in). The text is easy to read well-placed. Markers have a nice drop shadow effect that looks really cool.
The maps are larger
By default, Google Maps' maps are huge, taking up most of the screen, this means better printing, and One field You don't have to tab between fields to enter an address, just type it into the search bar all at once. Google Maps figures it out.
You'll find more parks, lakes, college campuses, and all the other landmarks that help you find your way.
Unlike MapQuest, you won't find a ton of clutter around the page. Most of the clutter on the MapQuest page is advertising. The only advertising you'll see on Google is if you type in, say "pizza" and the name of a town. You'll see flags around the map indicating pizza places. Presumably these are pizza places that advertise with Google. This kind of advertising makes sense.
Google Maps lacks some of MapQuest's features like "Save to PDA, and Send to phone, but to be honest those features didn't work so well anyway and Google will likely add features like these eventually.
You can check out Google Maps at http://maps.google.com