I like it a lot as it enables me to do this:
Dopplr is the first online tool for global business travellers. Dopplr lets you share your travel plans privately with a group of friends and colleagues whom you have chosen. It then tells you when people you know will be in the same cities. It also reminds you of people who live in the places you’re planning to visit.
Dopplr works on your personal computer and mobile phone. Dopplr is easy to link with most online calendars and social networks.
As announced at Le Web 3 in Paris yesterday, it’s now out of private beta and officially launched, and open to anyone to register and use.
If you want to selectively connect with trusted friends and acquaintances to get a view on where everyone is and is going to be, this is a good place.