dopplr v tripit: Use both!
Where dopplr's claim to fame is the social networking aspect, TripIt's is that it's a really nice tool to manage information: Take an itinerary (or a hotel booking confirmation), send it to TripIt, and all that important information will be extracted. You can then subscribe your calendar to it, or access your account through a relatively nifty iPhone application. No more searching for that hotel address when you arrive at an airport -- quite useful indeed, information at one's fingertips. It appears like you can also manage your entire meeting schedule during a trip through the application, though I haven't tried that.
Interestingly, TripIt's strengths don't really seem to be on the social networking side. Case in point, dopplr (like facebook) actively encourages finding friends and colleagues to share one's data with. TripIt lets its users walk their social network, and it lets them invite others by e-mail address -- but nothing in between (like a search by name).
Want to manage your flight data and hotel stays, and have a backup of all that travel information in the cloud? Go for TripIt.
Want restaurant recommendations or city guide pages that are built by the users? Better stay with Dopplr.
Fortunately, though, it's possible to combine TripIt's information management with Dopplr's social networking strengths: You can subscribe your dopplr account to the calendar feed made available by TripIt. The result: Dopplr gets the page views, TripIt does some of the grunt work till Dopplr catches up on that.
I'm having a hunch whose business model is going to survive better.
PS: I won't spend time expanding my network on tripit -- the one on dopplr is larger, I don't want to rebuild it, and it isn't too useful for the things I'll use TripIt for.