You can find a multitude of reviews and in-depth analyses of East of Eden on the internet or your local library so I'll make only three short comments.
First, one wonders whether readers will understand the title and much of the structure and symbolism of East of Eden in this age of biblical illiteracy. Sure, anthologies with excerpts of the novel will provide footnotes that will explain the story of Cain and Abel but that's hardly the same as appreciating on one's own Steinbeck's masterful reworking of the source text.
Second, Steinbeck's descriptions of the the natural world in East of Eden are exquisite. For example:
I remember the five-fingered ferns growing under little waterfalls, bobbing their green fingers as the droplets struck them. And I remember the smells of the hills, wild azalea and a very distant skunk and the sweet cloy of lupin and horse sweat on harness. I remember the sweeping lovely dance of high buzzards against the sky and Tom looking long up at them ... I remember the smell of crushed ferns in the creel and the delicate sweet odor of fresh damp rainbow trout lying so prettily on the green bed.I know only a few people as observant as Steinbeck was and none of them could put her observations into words like his. (His descriptions of human experience were also top-notch: "When angered she had a terrible eye which could blanch the skin off a bad child as easily as if he were a boiled almond.")
Third, Steinbeck's initial literary exegesis of the story of Cain and Abel Genesis 4 struck me as correct. It's only when he has one of his main characters go off and learn Hebrew that he makes that mistake common to those who know just enough to get themselves into trouble--the mistake of confusing word study with exegesis. Even so, Steinbeck's theme of the absolute freedom of human choice, even if overstated (moral agency is a better notion), is a refreshing cold drink in a day characterized by the soft determinism of heredity and social constructivism.
Steinbeck's re-expressions in the lives of the novel's characters of Cain's sacrifice and the deep anguish Cain felt at God's rejection should ring true for all people at all times. What Steinbeck fails to see, however, is that our moral agency, our freedom to do right even after doing wrong, is not always enough to regain a shattered fellowship. Even so, his accounts of sacrifice, rejection, forgiveness, and reconciliation--our common lot in life east of Eden--were powerfully evocative.
Steinbeck didn't get a Nobel prize for nothing. East of Eden is indeed a great book. A bit long for some tastes but my attention certainly never flagged. I can only echo pastor Ruffin Alphin: read it.