I have no problem with them, since I know my own english is kind of strange...

I had two Japanese friends that I used as a perfect comparison. One of them had PERFECT grammer and pronunciation, but you could tell she wasn't a native speaker in about a minute of talking to her.

The other had a horrendous accent, and her grammar wasn't all that hot, but you'd forget after about thirty seconds - because she had mastered the slang, and sounded like somebody born here, instead of there.

Go figure. :)