I put rootbeer in a keg once. It does take some soaking to get all the smell out, and you'll have to replace all 5 o-rings (dip tubs, QD's, and lid) as those will soak up rootbeer odor forever.
soaked the lid o-ring for a week in oxy-clean...still smelled like a can of Barq's.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10