His books were among the first that I read when I was a child (I remember "The Invincible" like I read it yesterday). They contributed to shaping my view of the world. They made me enter a science career. I met Lem once, in Berlin, when I was 15, and he signed one of his books for me. Later, when I went to Poland to study physics, I was happy to be able to read his books in the original language. I even started translating "Summa Technologiae" and "Dialogs" from Polish to English because there was no translation available (still isn't, probably).
And even though Lem became really (really) pessimistic in the recent years, I still think he's been one of the greatest writers of our times.
(If he'd preserved some of the optimism from his first works about the future, he might have explored alternatives to being dead forever, like cryonics... he even sort of wrote about it in "Fiasco".)
Rest in peace...