This has been a favorite book of mine since a seminary teacher introduced me to it in 9th grade. I read it first in high school, again in my early 20's, and now a 3rd time in my late 30's. It definitely helps that I'm reading Genesis now at the same time. It has helped me appreciate how much more the Book of Jasher offers. As always, there is plenty of the fantastical here that defies credulity--like Judah throwing an enormous stone in the air, catching it with one hand, and then sitting on it hard until it crumbled to dust--all to try to intimidate Joseph (who hadn't yet revealed himself) into not taking Benjamin captive. Some things like that are laughable, and when they put outlandish things like that into my favorite stories, it gets annoying. But aside from that tomfoolery, the book has much to commend it, and I believe it contains truths that are missing from the Genesis account.
My most memorable part: Abraham smashing his father's idols, and putting the hammer in the hands of his dad's favorite statue. When his father accused him of destroying them, he straightface told him that the favorite idol had done it, because he was jealous of the other ones taking a burnt offering that was intended for himself. When Abraham's father denied this was possible, because they are only made of stone and wood, Abraham had caught him--I paraphrase: "Then why do you worship idols of stone and wood that can neither speak nor act?"