Been meaning to read this one for a while, but I finally got around to Roadside Picnic by Arkady and Boris Strugatsky. I've heard a lot of good things about the S.T.A.L.K.E.R. games and this book is the basis for those, so I thought I'd give it a read.
My most basic top-level observation was the book's very dark, almost nihilistic tone... Roadside Picnic deals with a small number of characters, and those characters are very fragile... their numbers reduce significantly over the course of the story.
It's heralded as a great classic of Soviet science fiction, and I daresay it is packed with interesting ideas, from the merely mechanical ("empties" and "traps") to the philosophical ("the unlucky barber"). These ideas are presented in a range from obvious to subtle in the book, leaving potential rewards for repeated readings.
One great element of the story was its ambiguous ending. These can feel hollow or trite if done poorly, but I enjoyed it in this book.
Another thing that got me about this book was the tragic arc of the situation. The "zone" gets worse and worse, and so does the government's reaction to it. Eventually, people are forbidden from leaving, and those left behind suffer--the "zone" messes with them and especially their children. The area becomes more and more impoverished as people distance themselves from the "zone" and its inhabitants.
I also found it interesting how making the trade in "zone" artifacts illegal ensured that only the most unreliable people would offer payment for them. A lesson in bad economic incentives.
On the micro level, I found the descriptions of "zone" exploration vivid, creative, and tense. The characters were interesting and often surprisingly sympathetic. Even though mankind learned some useful things from the "zone," it was clear that, most of the time, what you got out of it was far less than you put in.