Just a tip from my recent experience.
I used Kayak to check prices for a ticket from San Diego to Rome in Feb.
It showed some great prices on Cheap Tickets and I think Expedia with Delta being about $300 more for the same flight.
I always check the Delta web site before I buy because sometimes they have flights with different times etc.
THIS time, to my surprise, the Delta web site ALSO showed the same inexpensive tix that Kayak only showed for the discounters.
Soooo I ended up buying on the Delta site and it was only $722inc. tax.
Kayak is good to find out who flies where but for pricing check the airline web sites before you buy.