My personal opinion, based on countless conversations with "the old timers", is that the millimeters never really faired too well in the US. That is just here in the US, I know the mm's are more successful in other countries. The one mm that took off like a rocket here was the development of the 7mm Remington Mag. Great cartridge, I have owned several.
The mindset here in the US, within the last 10 years or so, has painted a different picture though. I see an exhausting amount of 6mm and 6.5mm (pick a hybrid) in the long range shooting competitions. Nothing wrong with them at all, just not what I cut my teeth on.
The 8mm is a fantastic cartridge, and devastating on larger animals. As always, if you have good bullets, you'll get good results. If you have shit bullets, (you get my point). I've owned one 8mm Remington Mag, and have shot many trying to find one as accurate as the one I sold. So far, no luck.... If you find it, sell it back to me.