I've had only positive experiences with Japanese players. I've been in many roaming parties killing raptors with random Japanese people who showed no hesitation in letting me stay at all and were very friendly.
I'd hate to see servers divided by language/country, especially as populations are as low as they are right now and because of my good experiences with players from other cultures. If this was ever to happen, though, it would be around the PS3 release.

In the same way you'll meet bad mannered western players, you'll meet bad mannered eastern players. We're only human, after all.