I was a big fan of Kayak, but I want to warn everyone about a recent experience that has changed my mind. Our family lost $1000 in savings because of a Kayak alert failure.
I set up a daily alert for specific outbound and return dates. The alerts only last for 10 days at a time (highly inconvenient if you plan trips any reasonable time in advance, such as searching for a flight a month or more ahead of time - e.g. Xmas, three months in our case). I had it running (with resets) for about six weeks and saw only minor fluctuations in price. However, with my most recently reset alert, it failed to come after the seventh day (the email on the seventh one said it will expire in "three more messages" = 3 more days). It resumed again on the 10th day (last day of that cycle).
When I went to reset the alert, I did a check on Kayak's "chart" to see past price fluctuations and noticed that there was a $300+ dip very recently. After reviewing all my alerts, it became obvious that it occurred during the time frame of the missing alerts (represents over a $1000 missed savings for our family). Also, that alert on the 10th day and all subsequent email alerts from the reset now have a new format (i.e. look and feel).
Like most people, I'm pretty busy these days so did not specifically notice that the emails were missing as they hit my general email along with hundreds of others on a daily basis. I usually just give them a quick look as they come in. What's the point of having an alert if I cannot rely on it, but instead need to specially monitor if it will come and then do a manual search myself if it doesn't? It seems likely, if not evident that Kayak were sloppy and screwed up in its transition to the new format, but fat chance getting them to admit so, nor, if they do, getting any meaningful consideration for it, given their terms of service. (I have contacted them and they are "looking into it" - my paraphrase).
So now I am signed up with five other services (not just one any more)...you can bet that my next booking will be on those ones - I may still use Kayak just to search a route and validate prices, but that's about it.
I give it two stars because it is a good site to validate prices, and because I have saved money in the past with alerts, but I highly recommend you have alerts set up with multiple services because of a lack of reliability with Kayak (and my fear that other services may be just as unreliable).