It’s dead simple to use and it keeps you organized - all you have to do is forward confirmation emails to them when you purchase airline tickets, hotel reservations, car rentals, etc. Tripit pulls the relevant information out of the emails and builds an organized itinerary for you. You can send emails in any order, for multiple trips, whatever. It just figures everything out and organizes it.
I don't do that much regular travel now that I'm back in Kosovo, but this would have been handy while on speaking tours last year.