Historical Inaccuracies Make It Hard to Watch
When I was kid, I would enjoy watching this film when it came of television. Later as I learned more about The Battle of the Bulge, the movie became more of a very loosely based dramatization of real events with a focus on star power. Yes, Robert Shaw and Han Christian Blech were excellent, but that's no excuse for the story. I think the headline American actors Henry Fonda, Telly Savalas, Charles Bronson and Robert Ryan perform their perfunctory duties in this war film. Nothing more, nothing less.
I can overlook inaccurate use of tanks and all the other technical stuff. Shooting on the plains of Spain seems to take away from the overall German strategy of launching an offensive through the Ardennes Forest. It's hard to understand why the German tank columns couldn't just overrun Western Belgium in short order when watching this film shot in Spain. Why in actuality were the roads around Bastogne and Elsenborn Ridge so critical? The movie kind of glosses over this in the script and with the choice of shooting location.
If the movie presented a more realistic picture of the broader battle, I might have forgiven the final heroic scene where the Germans are repelled trying to gain access to American fuel supplies. Although the battle did not end in this climactic manner, it's a dramatization of how limited resources and time were working against the Germans.
Your first clue of how loose the script would be with details is the names of German and American leadership are totally fictional. No Colonel Peiper, No General McAuliffe, etc.
Basically if you like war films and are not interested in actual details, this film is passable entertainment. However, a battle this important in American history deserves an epic that portrays real events more accurately and diligently.