Edit: thanks for the recommendations, everyone.
Lem also has another essay about Philip K. Dick in his book Microworlds, you might check that out as well. He is a serious critic of science fiction, and he pillories the genre without mercy; Dick is the only science fiction author he considers worthy. He has zero patience with crowd-pleasers.
Still, he's got a wicked sense of humor. My daughter is reading "Memoirs of a Space Traveller" right now. It's definitely on the light, even funny, end. Start with the light stuff and work your way to the denser material.
I'd say Lem's strengths are his intelligence, his incredible pessimism (rare in a futurist) and his sense of humor, although you'll find only the first two in His Master's Voice, Return from the Stars, and Solaris.
"Return From The Stars" is also pretty good as I recall. Not a big fan of his early Tichy stuff though (it suffers from his own "Agatha Christie" kind of mediocrity IMSHO).
Of course, Solaris is considered his masterwork, for good reason.
[edit] Also, "A Perfect Vacuum" is a one-of-a-kind treat. And now that I'm thinking about it, "His Master's Voice" does "Contact" one better. (in fact it seems like Contact may have ripped Lem off pretty hard...)
[edit2] Damn, he did write a lot of good stuff. The short story "Non Serviam" presaged the Matrix and Greg Egan's Permutation City.
I've never been able to get into his novels, though. Mind you I'm not really a fan of science fiction per se.
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.
I admit that I don't know enough (I fail at reading Dostoyevsky) but it is counted as great writing despite lacking in the pace that modern readers desire.