I've been a guitarist for 30 years now, and I've been an AMS customer for over 5 years. I've bought a dozen guitars under their payment plan and have zero problems. It's true that some of their sales reps are friendlier than others, but I've actually never been treated badly. I see complaints online about transaction problems, but if you UNDERSTAND the arrangements and make sure funds are available at the right times, there should be no problems. Their payment plans are pretty simple, but some people just don't get it.
A good tip is; when calling these reps, speak to them how you would want to be spoken to. They're just working for a living too. I'll continue to shop at AMS for all of my guitar gear.