Case in point: I'm planning this weekend's trip from home to Sioux City. Google Maps routed the trip over I-90 and I-29, going through Sioux Falls, a total of 215 miles. Now, Sioux Falls is a very nice place, and I've got friends there, but it's a bit out of the way.
See that nicely marked highway running from Worthington, MN to Sioux City? It's Minnesota and Iowa Highway 60. (Iowa numbers its highways to match the numbers of adjoining state highways, where feasible.) It's a nice road, and 4 lanes for at least part of the way. I wanted to find out how much that would save me in distance. No way, in Google Maps.
Fortunately, I've got a mapping program for the Mac, Route 66. It's a little out of date now, but will do just fine for this function. After a bit of wrangling, I got the answer out of it: 157.2 miles.
I use Google Maps because it's up to date and works fine, but I wish it would do more.