That's such a good and such a difficult question @elfranz. My statement about the extremely dumbed down version of us is already very optimistic ;-) So as far as usefulness for studying the human brain, behavior and diseases goes, I think C. elegans is as far as we can go. And we can't translate findings on C. elegans one-on-one to humans, far from it. If we could, we would all live a couple of hundreds of years already.

Having said that, I'm sure of nothing. It's so difficult to draw definitive lines in nature; science doesn't even have a definitive definition of what "life" is, or what "consciousness" is. That's why biology, neurosciences and physics are the most interesting sciences these days, as they're really trying to answer the bigger questions humanity has had forever; who am I, why am I here, why is there anything at all, how did life originate, did everything have a beginning or was the universe always there...? According to evolution theory even those fungi and plants are relatives, but is a virus alive?

I know that wasn't a real answer :-) But thanks so much for asking it @elfranz; the questions are often much more interesting than answers ;-)