Just got six kegs, two last night, four this morning. Two pepsi, two 7-up, one root beer, one that was so well cleaned I couldn't tell. All of them cleaned up with zero leftover smell with minimal effort.
All got a 24 hour soak with 5 gallons of the hottest water my bath tub would put out and a scoop of OxiClean. Posts, poppits and lid went inside. Stirred extremely vigorously with a 5-gallon helical paint stirrer ($7) attached to a powerful cordless drill, stirred both right when the OxiClean went in and right before dumping it out. I had literally a 6" deep center vortex going on every keg. Dumped after 24 hours, scrubbed insides gently with a Chore Boy scrubber by hand (yes, my arms fit and I can reach the bottoms). Filled one up with the hottest 5-gallons of StarSan I could make, then pushed it from keg-to-keg (liquid to liquid jumper) with the CO2 at 30 PSI.
Literally zero smell, even from the stanky root beer keg. Total time, 2 hours labor.