I always thought it would be cool (actually, more extremely practical than cool) if you could change the route that Google gives you when you look up directions in Google Maps. Well, now you can! Don’t like the route Google gave you? Know that there’s construction on a certain road, or that traffic always sucks on a particular street? Just drag the blue line to the road you’d rather take.
It’s a little flaky right now, but give it time. The main thing is that it tries to route your “detour” back to it’s original route as quickly as possible, so you might end up having to drag different sections of the route to your preferred one to get it right. Hopefully it’ll get better (and since it’s Google, I expect it will).
What it does seem well suited for, though, is if you’re planning a trip you can easily plot side trips for sightseeing purposes. Maybe this is what the programmers had in mind, but to me it seems much more practical as a way of telling Google I’d rather take the Eisenhower to the west suburbs than the Kennedy today, because I know traffic out to O’Hare is going to be a bitch.