The m16 from vaportekusa is solid, but the 510 threading is very soft. I can only imagine that in time this will strip and there isn't much you can do about it. I try to be very careful when removing/adding anything to it and I can already tell that it has loosened. I really like the mod as a whole, but am glad that I have ordered something more solid and made of stainless steel. I don't imagine this will last very long. I have also found that the button backing itself out is pretty problematic as well. I find myself taking the switch off and tightening it back up at least two or three times a day. Aside from these issues, really not bad for a ~$20 mod.
BTW, this pic is from the fasttech PV2.
http://fasttechcdn.com/products/142/1423000/1423000-4.jpg
That is what the button on the m16 starts looking like. I do not think that is correct, and if your button starts looking like that, be prepared for it to just come out and the spring to go flying. If you see that, take the button housing off and tighten it back down. I think the button should be mostly recessed when the lock is off with just a bit protruding. If you can see the entire button, and extra space behind it, it's too loose. At least this is my experience with it.