This was one of the first books I read in Game Theory, and definitely the hardest. Those who want a gentle introduction to the concepts of modern game theory might do better with a simpler text such as Gibbons. That said, there is no substitute for quality. The depth of analysis is entirely necessary to get to the meat of the theory.
Osborne and Rubinstein write extremely well, softening the blows of some of the more complicated concepts. Their own substantial publication records in the Game Theory literature do much to recommend their version of analysis over others.