I think that our response to science fiction often brings to mind the danger of generalizations. We often say that science fiction is "less intelligent" or some other "less" than literature, but there are many exceptions, arguably because the generalization struggles to keep up with the variety of authors in science fiction.
We often tease science ficition for its flat prose. However, the writing of William Gibson's early work is clearly building on William S. Burroughs. China Mieville's imagery is fantastic. Sometimes we tease sci fi for its flat characters, but it would be difficult to read The Sparrow by Mary Doria Russel and not find her characters sophisticated. (Not to mention her themes.)
Gene Wolfe has suggested that science fiction and fantasy have a more extensive background that much of what we call "literary fiction" today and that these writers are building on the oldest forms of literature, suggesting that we should examine the walls that we place between one genre and another. Are the distinctions between genre arbitrary or overly rigid? In interviews for her books Oryx & Crake and Year of the Flood, Margaret Atwood attempted to distinguish between science fiction and speculative fiction, but even these distinctions seem precarious. Perhaps the best reference for an extended discussion that I can offer is Neal Stephenson's lecture at Gresham College in which he suggests that science fiction / speculative fiction, is the "last" genre. Click here.
The first respondent in this thread, Mshurn, has suggested that science fiction can be written to entertain or to explore serious themes. To be honest, I think SF tends to explore serious themes, but often gets caught up in the ideas at the expense of the prose or the character (Kurt Vonnegut's creation Kilgore Trout may be the most famous example of this conflict). However, as with any category of writing, there are exceptions to the rule.