"Defending someone's work is one thing, personal attack directed at an individual for expressing a critical opinion about either an artist's work or a particular style or technique, is another thing."

Well..it kind of depends what form the original "criticism" took, doesn't it? If someone, for example, said that another artist's music made them want to throw up, is their own music then fair game for harsh (but open and honest) criticism?

I rather think it is.