Overall this is the best tool I've found for keeping track of your final travel plans. Any booking emails you get, just forward them to Tripit and it's processed and added to the plan almost instantly. You can also set up alternate emails if you have different addresses or multiple planners. iCal integration is really nice.
Can be frustrating in a few ways: no calendar view for planning before you book, you can't attach a PDF receipt to your plans, the UI is old and awkward (but the mobile app is a bit better). However overall it does the basics better than anyone else so it's what I use every time.
Great for recording your final plans. Can automatically update plans quicker and more accurately than competitors.
No calendar view for planning a trip, web UI is awkward, can't attach PDF booking receipts/tickets to a plan.