I know this thread is super old, but it came up in my Google search so I'm still replying.
I'm no chemist, but I can say with certainty that the better it is, the more it hurts. I was googling because I just did a line that literally made tears just pour down my face it burned so much. But it looked really good. Perfectly clear. It has a stronger smell than I'm used to as well. About the time I got to this site, I got my own answer. My face went totally numb and warm. And I am incredibly jazzed up. I've done a lot of this stuff in the past year, so I've gotten a good idea of the quality range. It has been about a month since I did it last, so my tolerance is down. But last time I did some after about the same time and barely felt it because I had been on a long long binge before that. So it's safe to say the burn is a very good sign. I honestly have gotten more of a rush off of snorting a line of the burn-y stuff than shooting a large spoonful of more mid-grade stuff. My legs are still entirely numb right now.
I'm sure there are rare occasions where they cut with something weird that burns, but the meth itself burns more in higher concentration. Also, if you're buying some and don't have the opportunity to test it out, look for clearness of the crystals and SMELL IT. I've had people say meth has no smell, but every time I've gotten really impressive quality crystal, it's had a strong chemical smell to it.
Hope this helps some other people. Now I'm going to run 30 miles or something! :D