One of the challenges to getting value out of this is having friends share info with you. Here's a tip. If you add contacts, select "all contacts" and Latitude will start by showing you all of your correspondents who are already on Latitude (and presumably require less convincing that this is worth doing):
That brings up this screen:
Click the names of the people you want to add and you're good to go - now you'll be able to see where your friends are hanging out.
Last thing to do with Latitude? If you have a blog, add the Latitude badge to your blog so people can see where you are. I've added it to my blog, and this is what it looks like:
The Latitude Badge site includes a one-click add to Blogger, or you can copy the embed code to add it to whatever platform you use. Easy!