great movie

It's based on a book by Toivi Blatt (he's the teenager who is Schlomo's assistant in the shop). AFAIK, it's pretty accurate although some specific details were changed for dramatic effect -- for instance, they show Sasha and Luka talking quite a bit whereas in reality they both spoke different languages and didn't really say much to each other. Actually come to think of it, all the prisoners speaking a common language is probably the most significant deviation from reality in the film.
Other scenes like for instance, Schlomo killing several Ukrainian guards, while it may have happened, does not strike me as something that was put in the film because it happened but rather for dramatic effect. The way the killings are discovered (Chaim leaving blood stains on the handle) is also seems made up -- I don't think it's known how exactly the killings were discovered.

Apart from that, I believe the abuse of the prisoners, the plotting, the execution of the escape in general are pretty accurate.