It is well-documented what amazing technical feats the POW's were able to accomplish in the stalags. There was even an entire section of the British Secret Service dedicated to coming up with all sorts of clever ways to send these captured men the tools they needed to facilitate their escape attempts, i.e., sandwiching maps between the split sides of a record album (yes, the Germans allowed the prisoners to have records in the camps) or compasses in pens. At Colditz Castle, one of the more forbidding stalags, (actually an offlag since is was for officers only), many, many tunnels were dug and disguises created. One man actually created a German sergeant's uniform totally from scratch, donned a moustache and created an overall impersonation so realistic, it fooled two out of three sets of sentries. Some of the POW's built and concealed an entire glider that would have carried two men off the roof and over the wall! The only reason it didn't fly was because the prison was liberated before they got the chance! The Colditz experience is well documented. There are many books written about that particular prison complete with photographs, including one by a German officer confirming these amazing escapes and attempts. The reviewers who doubt what can be done when necessity is truly the mother of invention should look for them and learn something.
As for the prisoners not being in jumpsuits, as suggested by one reviewer as one reason to question the authenticity of the film? Ludicrous, POW's wore what they were captured in. The German military (different from the Gestapo and the SS) considered them soldiers and allowed them to keep their badges of rank.
As for the film itself, it is long, but absorbing. There are historical flaws (as there are in all movies), but several of the former POW's participated in the filming process, keeping it, for the most part, very authentic. As for the emphasis on Americans, it's true they were not among the escapees per se, but several did assist in the effort before they were transferred out, as mentioned by a previous reviewer. However, you must remember that the movie was made for an American audience in 1963, long before international distribution revenue became so important to a studio's bottom line. They needed American stars who would appeal to an American audience. Who knows, perhaps if they were to remake it today, the cast would be all British and German, but I doubt it (see "Hart's War" where not only the plot, but all the British and Canadian characters that were in the book, disappeared).
All in all, "The Great Escape" is an entertaining movie telling a fascinating story of what ordinary men can achieve in adverse circumstances. It's well worth the time.