When well written and acting in a Joker-ish manner, always.
I could never hear Morrison's Joker a Hamill though, which is not to say he's not well written, just not written as any take on the Joker I can properly relate to. I had the same problem with The Dark Knight movie, Heath Ledger gave a powerhouse performance as a psychopathic monster in the role, but it never really felt like the Joker to me.