Interestingly, I've had more problems out of guns I purchased new (Ruger MkIII and Remington 597) than the more numberous firearms I've purchased new.
That said, I generally only buy new now. About the only reason I would go used at the moment would be if either a) they don't make what I want anymore or I can't find one new or b) I got a really sweet deal. Most of the time, used guns aside from surplus stuff aren't really that much cheaper than new. Usually around 10-20% from new.
I also tend to have somewhat odd preferences in firearms. Usually I'll go into a gun store looking for something only to find they only carry longer or shorter barreled ones rather than the size I want, only have stainless if I'm looking for blue, only have railed ones if I'm looking for one with a carry handle, etc. I'm at the point now where I will only buy exactly what I'm looking for and not what happens to be in stock. As such, I get most stuff special ordered.