The United States Sentencing Commission has released a Report on the Impact of United States v. Booker on Federal Sentencing (caveat, the PDF is 277 pages). Professor Berman at Sentencing Law and Policy has an initial post on this here in which he states:
Also based on a quick scan, here is a notable sound-bite: "The severity of sentences imposed has not changed substantially across time. The average sentence length after Booker has increased." USSC Report at p. vii.
I have not had a chance to read this report, as I'm preparing for a trial, but took a short break from work.