I have 2 M tanks that were ordered through vaperev over 2 years ago. They are made of Makralon and they are still just like new, after running countless numbers of the "caustic" liquids through them. I think most people don't like them because they are about 23mm instead of the Kayfuns 22mm, but I never minded that, and they aren't cheap...
They're cheap at Kidney Puncher. And I agree, they're extremely tough.
Andria