Entirely used. I would never buy lenses new - simply a cost issue. If it's something that doesn't deteriorate much over time, I'll buy used. Items like computers and bodies that go out of date every year - I buy new.
I have had some incredible deals on lenses just because they are cosmetically a bit battered. Just make sure you get the seller to get very clear pictures of the optics and make sure you trust them to take the lens back if it's not up to your standard.
All of mine are new, but lower end lenses since I can't afford top end lenses.
If I could look at a lens in person to see how it is, then yea, I would buy used.
Otherwise, if online, you don't know how legitimate the claims or even the pictures are.
Maybe my vote is skewed a bit, because my camera body came with 2 kit lenses....out of my 4. I bought the 35mm 1.8 new, because it's just $200. But I got the 10-24mm used for several hundred cheaper than new. I don't think I'll buy a brand new lens again. It's like buying a brand new car versus one right off lease.
Almost if not everything I use is new for my Canon setup - but I've got a fair few second hand items and lenses around for some older gear so mostly new. Given the choice I try to go for new where I can.
Only lens I bought new was the 18-55mm kit lens that came with my D5100. Everything else I've bought used (or refurb). That's nine lenses bought used so far (though I don't still have all of them) and out of the nine, only one did NOT perform at the level I'd expected from that lens, but then that lens had cost me about $40, so I probably shouldn't have expected as much from it as I did. That was also one of the first extra lenses I bought, and the $40 was well worth the lesson to buy quality and do your research on a lens before you buy.
71% Used. Two new: AF 24/2.8, AF 50/1.8 Nikkors; five used: AI-S 105/4 micro Nikkor, AF-S 55-200 kit, and the three that came with the FT3 - Vivitar 28/2.5, Nikkor-S Auto 50/1.4 & Vivitar 75-205/3.8 Close Focus. The 155-200 replaced my 18-135 kit lens which I gave to son #2 when his 18-55 broke.
I'm at 50% for working bodies: the N90S and the Nikkormat were used, while the D40 and Minolta rangefinder were new.