I agree that Google Now keeps getting better and better at displaying relevant travel info, especially on day-of/before travel. I just recently started seeing a card that collates my entire itinerary instead of just showing individual cards for flight/car/hotel/etc which is another good step forward. During travel, I'm using TripIt much less these days.

The main drawback of JUST using Google Now is the lack of manual control... you can't take a look at an itinerary a week out, for example. Another big drawback is that Google Now doesn't seem to pick up on cancellations... I often make plans months out, especially hotels, and then change them multiple times in the time in between... this leads to 3 or 4 false cards on day-of-travel because it's displaying EVERYTHING I've booked, including EVERYTHING I've also canceled. This tends to be an even bigger problem with leisure travel as I book and re-book as I incrementally find better deals/values or refine dates/destinations/plans.