I don't really mind obscene user names, and I'll admit they can be charming if they are fairly clever, like Dixie Normous mentioned above.
I myself managed to make one by accident. I have common MMO character I make called Nipakhet, It is an attempted combination of Neith and Pakhet, when I realized using those 2 as a First name Last Name combo would not fit in the character limit. When it came time to group up with others for dungeons, and character names were shortened for ease of communication. EVERY GROUP, was quick to call me Nip or Nippy and make every joke possible out of it. So I went from having a character named after 2 war goddesses to a character named nipple.