View & filter destinations on a map, with options for flight cost, weather, destination preference, flight times & activities.
Pro Able to search bookings concurrently on multiple travel sites
Kayak searches hundreds of travel sites to find all the possible flights to ensure you don't miss out on the best options. They have the most comprehensive results out of the competition.
One of the biggest advantages of Kayak that most of their competitors don't have is the choice of where to book after you have found what flight or hotel you want. This means you are not locked into a single vendor and can compare the various booking fees etc.
Pro "Explore" is awesome for trip planning
Kayak has a feature called "explore" that lets you answer the question "Where can I go for how much?" Explore is a fantastic tool for initially planning out your trip while finding the cheapest flights.
You punch in parameters for a trip and how much you're willing to spend to fly there, and your options magically appear on a map. You could punch in, for instance, that you want a maximum-four-hour nonstop flight in January to someplace where there's a beach and the temperature is between 70 and 90 degrees and they speak Spanish, and you don't want to spend more than $500 on airfare; Kayak will spit back your options.
Con Dishonest about prices
There are a large number of complaints against Kayak questioning their honesty. It's frequently reported that after booking on the site and enter your credit card information, you will be told to call Kayak as something went wrong. You are then given a higher rate, or sometimes charged the higher rate without being told.