Ebert's take focuses on the human aspect of Solaris, but the other aspect of both the movie and the book that is just as appealing to me is our inability to understand the alien ocean itself. All the attempts to anthropomorphize its behavior fail, and in the end Solaris research is abandoned because the ocean is essentially inscrutable to humans. This theme is also covered in another Lem book, "Fiasco."

I love the idea of an alien, presented before us, completely available to us to interact with, and we cannot bridge the gap because we and it are so different.

It just leaves you with "What is it doing?!" which is such a wonderful contrast to the deep emotional impact that its behavior (creating the doppelgangers) has on humans. Does it know? Is it coincidence? Is it malicious? So many good mysteries.