2 of my 10 kegs were purchased new. I got the new ones for about $75 apiece on eBay when I first started brewing. In my opinion the new kegs look better and the fit and finish is much better. The parts fit together better, seal better, etc. There's no messing around with them required at all to get them to seal.
With the used kegs I feel the various parts need to be mucked with before they seal properly. When I pull a gas line off a used keg, for example, the poppet will continue to release gas until I fiddle with it or give it a whack with a wrench. I often have to re-seat the lid a couple of times before the keg seals. The used kegs do work well but sometimes they require a little convincing before they behave properly.
I'd love to have all new kegs but for me the price difference between used vs. new isn't worth it. I don't mind fiddling with the older kegs to get them to work and I don't care if my kegs look like they've been in a car crash. However, if the price of used kegs goes up or I find a good deal on new kegs, I'd pop on the new kegs any day.