John's instruction is first rate. His wife Vicki is probably the best instructor I have ever worked with. Her ability to diagnose a shooters issues and help them improve is a sight to behold. Taken together, they are a super pair.
John offers a whole system. Its not about shooting, really. Its about not shooting. He emphasizes situational awareness, disengagement, strong verbal challenges, and movement. If all this fails, then accurate, sustained gunfire is taught. The courses are long, well thought out, and physically taxing. You will literally be on your feet for 12 hours. Plan for that.
Another key focus of John's is gun agnostic skills. They emphasize solutions to issues that will work with any gun, regardless of what you happen to have in your hand. As he would say "there is no guarantee that the gun that gets kicked to you through the puddle of beer and blood is going to be your favorite type". There are lots of excerises where you have to use other students weapons. This became less useful a couple of years back when 80 percent of the guns were glocks, but now there is a true variety again.
Finally, do not bring pretty, safe queen guns. Your gear is going to get hot, dirty, dropped in the mud, and will need to be able to shoot hundreds of rounds without cleaning. Leave your shiny pretty toys at home.
If you can get to a DTI course, do it. You won't regret it.