I only buy new from reputable dealers. That way I get a warranty and customer service, and if I ever decide I don't want a lens I'm usually only out a little more than the cost of a rental. Plus, who knows what used lenses have been through. There are youtube videos about how to take lenses apart to clean the internal elements:
What if someone did this to a lens I was buying used? I really don't see the point of buying used except if you want a vintage or discontinued item. Just my opinion.