Perhaps the most useful feature I have found with the tool is the ability to pull up a graph showing changes in airfare depending on departure date and trip length. Picking up with our hypothetical Atlanta (ATL) to London-Heathrow (LHR) flight, by clicking on the bars just below the flight return date, a calendar and bar chart appears.
For the trip to London, the highlighted darker blue bar shows us that leaving on Sunday would be one of the most expensive options. If you could leave two days earlier on Friday or a day later on Memorial Day, there are cheaper flights. Note below the chart are adjustments for the duration of the trip as well as tolerance in terms of +/- days in the length of the trip. In the calendar, the actual lowest fares for that day are displayed, again with the cheapest options in your calendar current view highlighted in green.
Another feature that I like is the map view. If you click on the small map, it will expand to show your flight path.
The map also shows other possible destinations with a red circle labeled with the fare for a flight from your origination city - ATL in this case. If you click on a new destination, your fare bar chart (now smaller in the top left corner) updates to show data on the airfare to the new destination. You can go back to the larger bar chart by clicking on the smaller chart.
Zooming in on your destination is a useful way to fine tune your search for destinations with multiple airports or potential nearby alternatives. Let's use Atlanta to New York City for example.
I have zoomed in on the New York City area. I then clicked New York City and now I see dotted lines to the three major airports in the New York area: Newark-Liberty (EWR), LaGuardia (LGA) and Kennedy (JFK). The data shown for New York City overall includes flights to these three airports. You can now click on each airport individually to get the a specific, detailed fare chart to that airport. You can see that Westchester County (HPN) and Islip (ISP) are also visible in this view. The fares to HPN and ISP are higher than the three major NYC airports but they may be a better alternative for your trip.
I hope this overview has shown you the usefulness of Google Flights. I know that I have just hit on the basics and as I learn new ways to use the tool or find informative articles about it, I will post or tweet them. Be sure to follow me on Twitter at @DebrianTravels! I know I am anxious to use Google Flights to plan a real trip in the near future!