I have not used Google Flights to plan an actual trip yet, but I have played with it several times and it definitely doesn't look like other travel sites I have used. It allows you to gain insight into available flight options in a unique and intuitive way. I do think it is a hidden gem because I have recommended it to several people that had not heard of it.
Let's assume I wanted to search for roundtrip flights from Atlanta (ATL) to London-Heathrow (LHR) for the Saturday before Memorial Day. Above the search box I can choose which cabin I want - first class please...if someone else is paying! - and the number of people in my group. I can specify the maximum number of stops I will accept or the most I am willing to pay. I can limit results to a specific airline or airline alliance as well as restrict the departure or arrival times. In the "More" dropdown, I can choose a maximum flight duration and connecting cities.
The search results are organized into "Best flights" with other options below. The price of the cheapest itinerary is highlighted in green. The WiFi symbol beside the flight duration indicates that the the flight is operated with WiFi-enabled aircraft. A symbol with full bars indicates available WiFi on all segments, while partial bars lets you know that some segments are not WiFi-enabled. Click on the flight to get more information (see below). You can even change the currency in the page's footer!
Google describes "Best flights" as offering "the best trade-off between price, duration, number of stops and sometimes other factors such as amenities and baggage fees." Since your idea of the best flight may differ from Google's, you can click on any flight to get more detail on aircraft type, leg room (in the cabin type you searched), and amenities. For Virgin Atlantic 104, you can quickly see that the flight is operated by an Airbus A330. The average legroom in economy is 31" and the aircraft is equipped with WiFi, in-seat USB power and on-demand video. I really like the ability to quickly click between multiple flight options because, as a plane geek, I am often trying to get on - or avoid in some cases! - a specific aircraft type. If you don't want to select that flight, click the "X" in the upper corner and choose another. Once you find the flight you want, you can then select your return flight below.
Note, you do not book your flights directly with Google. Once you choose your return flight, you are presented with several options of how to proceed. There are links to the airline web site, airline partner web sites, and various third party online travel agents. You can also save your search or share it with others.
It's the booking stage where Google Flights can pose some difficulties.
Personally, I prefer to book directly on the airline website. I have used sites like Expedia in the past but mainly for hotels and rental cars.
While I like the clean layout and ease of reviewing flight options, if this was all Google Flights had to offer, it would only be marginally more useful than existing booking tools. It's the extras that really make Google's flight planner shine! I will review these features tomorrow.