I purchased a refurbished 7D and am currently watching prices for refurbished 5DIII's to drop to grab another one. I'm not, yet, in the market for another lens, but I will definitely look at Canon's refurbished lenses first.
Canon's refurbished products must pass individual inspection before being sold. I would assume that this includes a focus check for lenses. New lenses don't get this kind of individual testing before being sold. Now that refurbished camera's and lenses (through the Canon store) have the same one year warranty as new products, I don't see a down side for buying through Canon's Refurbish store as long as the price is right. (I have, occasionally, seen some promos on new products beat the pricing of Canon's refurbished pricing.)