I actually traded for G20, and I will say that I was truly impressed with it being used. You may find some really great deals on any used firearm, I guess it will depend on where you will be purchasing it from. Used may save you a $$$, but you know that no one has abused a new firearm.
For the most part I only buy new. With that said about 1 1/2 years ago I purchased a second Glock 23 with night sights and three mags. The gun only had about 100 rounds through it and looked brand new. I ended up picking it up for $400 and so far have over 12K rounds through her with no issues. As others have said, know what to look for and ask questions. If you can get a deal on a good Glock you've been looking for don't hesitate to pull the trigger.