Interesting, remarkable for its subgenre, credible… but I didn't find it funny.
I'm not a specific admirer of mockumentaries, but I recognize their value if they're funny. The film reports on the tour of a British rock band called Spinal Tap, and shows the enormous difficulties and crazy things they carry out on and off the stage. It's supposed to be a comedy... but, to be honest, it didn't make me laugh.
I recognize the value that this film had for the cinematographic subgenre it launched, and the interest that the film has for cinema students and others who deepen their knowledge of the seventh art in greater detail. For me, as I'm just a guy who watches films because he likes them, it's different: it's harder to convince me to watch this a second time because of the many technical arguments they might use. Being a comedy, it has to be funny. If it doesn't, it failed as a comedy (even considering the fact that I may not be the target audience, that would just be a sign that it's not a film for me).
Although it didn't make me laugh, I recognize that Rob Reiner does an interesting job and manages to give his film enormous authenticity on all levels. I wonder what fieldwork he did to prepare for the project, whether he spoke to journalists who follow the music industry, with bands or music artists, because in fact the film captures quite well the bizarre things that can happen on a rock tour. And the work of the main actors (Michael McKean, Harry Shearer, Christopher Guest) is equally worthy if we consider that much of what they say is improvised at that moment, not previously written. The film looks cheap and this is perhaps even intentional: the cinematography resembles a “found-footage” film, with the image shaky, poorly calibrated, full of grain at times. The sets are very good and the soundtrack, made for the film, is absolutely believable.