As a Japanese, it's INCREDIBLY ANNOYING to have someone walk up to me and throw random terribily placed, otaku-flavored Japanese words, in absolute terrible pronunciation, and completely out of place in context.
In those times, words like 'weeaboo' does cross my mind.
You wanna hear genuine "Japanese and English mashed up" conversation, just watch me talk to my mother and sisters.
Then again, I'm also terribly annoyed when I correct someone or provide information to be called a weeaboo.
I understand you can't see "race" through the mask of the internet, but seriously, STFU.
As for the usage of the word.. In general any insults are obviously not going to be plesantly recieved, duh.