You mentioned in another thread that your budget is rather limited. I didn't even realize you could get a used Gammill for just 3-4000. I've used a Gammill several times, but the one I rented time on was one of the top of the line 30,000ish models. I liked it, but not as well as others that were much, much less expensive.
The dealer from whom I rented time had problems several times with the tension. Once I had to stitch down some loops in the back that we didn't notice until it was too late to go back and another time she only charged me for supplies (batting and thread) rather than the quilting time because I had loops in multiple places on the back. It worked out okay for me because that one was just a scrappy for around the house. She actually rescheduled another time I was supposed to use the long arm because she was waiting on a part to fix the tension. I think tension sounds like one of the biggest problems with multiple brands of machines though so that might be normal.
If this one is in your budget and is in good condition, you're probably getting a really good deal. I hope it works out for you! I understand your worry about spending so much money. I'm struggling with the same thing. It really is a lot of money, but Gammill certainly has a good reputation.