Before watching the movie:
Mockumentaries tend to be great or mediocre. CSA: The Confederate States of America was disappointing, but mostly because I didn’t expect the direction it took, and it had a much stronger message than I imagined. As a comedy, especially one that is a cult favorite over 25 years later, I expect good things from This is Spinal Tap.
I’m not much into heavy metal, but the point is satire, so it should be pretty good. Also there are a lot of big names and big non-names (you may not recognize the name Harry Shearer, but he’s one of the core actors on The Simpsons), so a lot of star power and star talent.