My recent experience with Kayak was quite poor. My United flight to DCA was delayed 2 hrs which meant I would have missed my connection to LHR. Luckily, I reached the airport early and UA offered me alternates that worked. Kayak didn't notify me about the delay until 3 hrs before the original departure time. Last week, I got a text message from the airline 7 hrs before departure that the flight was delayed an hour. 3 hrs before departure, Kayak still said it is on time (it wasn't, was eventually delayed 3 hrs due to LHR issues).