I bought a used HQ16 almost 3 years ago. It works wonderfully. I've had some 'not so fun' days with it, but it usually turns out that the problem is ME, not the machine.
Mine is paired with a pcquilter and a frame, so I do not do free motion quilting.
Ask about the maintenance of the machine before you purchase it. Like I said, I've had no problems with my HQ16, but it was well maintained before I got it.
The person I bought it from was upgrading to an HQ Avante - which made me feel good about the reputation of the company and the HQ16. I figured she wouldn't upgrade to another HQ machine if the HQ16 was a lemon!