There have been lots of threads on expired films and processing of old films.

Obviously film doesn't "go off" on the day it expires, and, while it can remain in good usable condition much longer, it depends entirely on storage conditions.

Personally, I wouldn't buy expired film (the only time might be if I wanted to experiment with a discontinued film), but once fresh film is in my freezer, I don't worry if it then goes a couple of years over. I think that perhaps, having used limited leisure time and hard-earned cash in travelling somewhere photogenic, I don't want to risk disappointment by trying to save a few pennies on film.