Every time I read a book by Phil Dick, I'm surprised. How did he come up with this stuff? You get repetitive themes: alternate realities, psychic phenomenon, alienation, a constant questioning of the nature of reality, and so on. But he managed to make it fresh and exciting nearly every time. And if the uniqueness of his prose and plotting isn't enough, he off-handedly peppered all his writing, especially his best, with interesting thoughts, bits of philosophy, and keen insight. Granted, the man's no philosopher, but he'll still get you thinking.
Ubik as a particular manifestation of Dick's psyche is no different. From a few chapters onward, Dick continuously keeps us guessing, trying to figure out what the heck is going on, what Ubik is, and why reality keeps slipping out from under our feet. With almost disgusting ease, Dick manufactures worlds, situations, people, and technology that, though slightly dated faced by today's hyper-aware (of itself, science, theory, fad psychology, what-have-you) sci-fi, nonetheless flawlessly convey something true about man and his relationship to a rapidly changing (some, including Dick, might say disintegrating) world.
What is Ubik? How safe is it? Is Glen Runciter really dead? Why do all the objects in the book keep morphing into earlier technologies (so that what is a state-of-the-art stereo one day is an old phonograph the next)? This book will keep you guessing until the very last page, building up new theories of what's 'really' going on only to dash them to pieces a few pages later.
If you like PKD and haven't read Ubik, get it now. It's one of his best. If you haven't read any PKD, Ubik is a good place to start. Though it's a little disorienting at first, especially if you aren't familiar with his fascination with psychic phenomena, the story quickly grips you, and will also introduce you to most of his major themes. Great, great stuff.