Tamara Glenny has a pretty good rant about Google Maps directions (and by extension, others of their type):
What makes for good directions? Bad ones are pretty easy to identify. When we got into the car and headed for D.C., my boyfriend handed me the Google instructions he’d downloaded, using the basic method of inputting our address in Brooklyn at one end and that of the hotel in Washington at the other. This produces a list of indicators (“I-95/New Jersey Turnpike to exit 1,” or whatever), with each stage annotated with varying distances (“0.3 miles”) and times (“93 minutes”). The semi-uselessness of this style is fairly obvious—the main one being the impossibility of knowing whether Google’s 93 minutes is remotely close to one’s own, especially allowing for traffic interference (or the lack thereof). Besides, only anal-compulsive types who endanger their lives by looking more at their odometers than the road can keep on counting the 0.3 and 1.6 mile chunks you’re supposed to monitor.Now that you've pointed that out, they might.
We also got the distinct feeling that the Google Map algorithm tends to go for highways and straight lines—whether or not they really get you there any quicker. When we told our hotel receptionist downtown that the map had sent us via Silver Spring, he just laughed, rather pityingly.
I find that when you’re driving in New York City it’s important to know, say, that you stay on a street until it passes under a railroad bridge, after which you take the next possible sharp right turn. That way, you’re alert, you’re ready for the turn, and you’re looking at the road and what’s around it, not at the bloody odometer waiting to see it click from 0.2 to 0.3.
There’s nothing in a Google Map instruction to let you know that when you get on the Long Island Expressway at a certain spot in Queens intending to take the next turnoff for the Brooklyn-Queens Expressway (BQE), you have to do an instant zero-to-60 dash from a left-hand ramp to a right-hand exit across four lanes of traffic. If you delay, you either miss the exit or you’re dead (or both). So the instructions I write for such situations have a lot of detail. If you’re being told to take the Fort Hamilton Parkway exit off the Prospect Parkway, it helps to know that you’re going to pass a completely different Fort Hamilton Parkway exit several miles earlier on the aforementioned BQE and that’s not the one you want. Think Google Maps is ever going to tell you that?