Can verify the history of the guitar? Is it exactly what you are looking for? Is it the only way that you can acquire a specific make/model you're after?
All of these questions and more cross my mind when looking through used guitars. Like anything used - it may or may not come with a whole new world of problems to solve, whereas a new one should have some sort of warranty to protect you against some of the things that can go wrong.
I bought a brand new Gibson SG and found that the neck wa cracked and got it replaced free of charge. That just doesn't happen when buying used.
As for other guitars - go and play some - find what fits and feels good in your hands, it's the only way to be sure.
D.