Britney's attempt to enter the Twittosphere sadly fails as it is clearly just another PR stream from "her people" as Gary Vaynerchuk points out. The most important thing is authenticity. Either do it yourself or don't bother.
Two examples of famous people getting it right are Stephen Fry who began Tweeting 10 days ago on 9th October, and has gathered over 7,000 followers. What is impressive that despite his schedule, currently filming in Uganda, he manages to follow very nearly all his followers and even answers questions in real time, occasionally.
Scottish Tennis star, Andy Murray, currently ranked 4th in the world has been on Twitter since April 4th '08 and seems to be both authentic and mobile, tweeting from a 'Twitterberry' although he only follows just over 4 percent of his followers.