I second the recommendation of The Cyberiad... especially for people interested in computers, robotics, philosophy, and artificial intelligence. It's really brilliant -- and a very easy, fun read.
In addition to this, I recommend A Perfect Vacuum and Imaginary Magnitude, which both fall into the category of "books of reviews of books that don't exist".
I've never been able to get into his novels, though. Mind you I'm not really a fan of science fiction per se.
I am a fan of science fiction, but haven't been able to get in to his novels either. I think they're boring. His best stories, by contrast, are hilarious and very fun and easy reads.
Perhaps the issue is that, like so many science fiction authors, his ideas are interesting but his actual writing, characters and dialogue have nothing in particular to recommend them. In short story form, the ideas come and go quickly enough that one doesn't have a chance to get too bored with them. In a novel like Solaris, on the other hand, there's really only one idea and it takes a hell of a lot of words for it to actually get out there.
Asimov is similarly bad at actual writing, but his ideas are sufficiently good and they come sufficiently thick and fast even in his novels that he's still pretty okay to read.
In contrast an author like Neal Stephenson manages to write a science fiction story with proper characters who are interesting to read about even when they're not expounding on some science-fictional idea.
It's harder to tell how good an author's writing is when it's in translation. The Cyberiad's writing was absolutely fantastic IMHO. But I can't say if the Polish original was that good. Lem's longer, more serious novels don't sparkle like that, at least in the English translation.
True, though good writing isn't just about good prose and putting well-chosen words in an appropriate order, it's also about more macro-level structure and tension and pacing and characterization.