I've been using it a few months (May 2010) a find it a very interesting service.
Just checked my history and I've clocked up 8,922 miles so far.
The dashboard (google.com/latitude/b/0/history/dashboard) is very nice and allows you to playback journeys or your travels over time and gives you a nice breakdown of where you've been work/home or out.
I also like the fact it follows you from country to country (if you have data roaming on - v. expensive).
However I do recognise the fact it can be seen as one step to far for "Big Brother", and could be used by some to stalk people. i.e I do not recommend it to be used by people who are not comfortable with people knowing where they are.
So far I have a good few friends family and co-workers on it and we've had no major issues.
All in all I would recommend it to the curious and those who wish to be at the forefront of location based services.