I have not done one yet, but from what I can tell from talking with fellow authors they do not seem to effective.
If you decide to try one, There are plenty of companies out there that will run tours for you and have all the contacts in place. For example, I have been told this one is good:
Reading Addiction Virtual Book Tours
http://readingaddictionvbt.blogspot.com/
Still considering trying one just because I have not yet. I do not see how they could HURT that is for sure.