Their travel account monitor is excellent with all of the details I see at the airline's website appearing right in TripIt. They also have mobile clients for iOS, Android, BlackBerry, and Windows Phone. I personally find the MyTrips app on Windows Phone to be a bit better.
The one thing I wish TripIt could do is help me research and plan trips, but it is more of a trip manager than a planner so you have to look elsewhere for this functionality.
TripIt allows you to connect with other TripIt members and share trips while comparing trip stats in a sort of competition. You can share specific trips through Kayak, but I don't see support for a Kayak network of friends.
Kayak's trip planning functions are pretty amazing and the information they provide on airlines, hotels, rental car agencies, and more is invaluable. There are mobile apps for iOS, BlackBerry, Windows Phone, Android, Symbian, and a mobile optimized web page.
I would probably go all in with Kayak, but the price monitoring service provided by TripIt Pro is way too valuable to me.
I never gave much thought to fluctuating airfare prices and there is no way humanly possible you could sit and track fares every single day by yourself. Thus, automated tools like Yapta are invaluable. One thing to keep in mind is that these airfare tracking tools generally only work when you purchase your tickets directly from the airlines and not through a travel agent or site like Travelocity. If you do use a 3rd party you will need to work with them on getting refunds. With apps like this and Kayak though you should be able to easily plan and make your trip reservations.
Actually, as you browse around the Yapta website you will notice that the trip planning and search tools are powered by Kayak so you do get both services in one through the web browser interface. This partnership of services is not yet supported on the iPhone application though and I also want to see at an Android application soon.
I am still discovering and figuring out the best travel apps and services for myself so if you have a favorite please do share here with me and the readers.