Do you have a live Internet connection when evaluating your Quantity expressions? The new units system relies heavily on WolframAlpha for interpretation of units, and many Quantity or other unit-related functions will fail if Mathematica is unable to contact WolframAlpha for help. This seems to happen mostly when entering Quantities; if the strings representing your units are not immediately recognized by Mathematica, the strings get shipped to W|A. I've gotten in the habit of starting an inline W|A box, typing my desired quantity, then using Evaluate In Place on the box. This gives me the quantity in a form that, I believe, will not require W|A next time I evaluate the notebook. Using InputForm will also let you see the proper naming and spelling of the units for next time.