A film I appreciated and enjoyed more on a second viewing. But, while it's clear how influential it was on other film-makers, for me it feels more dated and awkward than Antonioni's earlier films. I know, I'm probably in the minority on this one.
Antonioni switches back and forth between 'plot' and abandoning it to shoot a now tame and silly three-way sex sequence, or a rock group stiffly performing. (Yes, that arbitrariness is part of the point, but its a simple point we get pretty quickly). There is that great sequence in the middle, when Hemmings starts to 'discover' the hidden images in the photos he took, but other than that sequence, this didn't have the visual brilliance of other Antonioni films that offset the sometimes simplistic characters and ideas.
That's a running feeling I have about Antononi - he's often simply judging the 'shallowness' of society and his characters instead of getting inside it, And he repeats himself on that theme obsessively. It's also sort of hypocritical. Clearly, Antonioni is judging his character's obsession with sex and material things, yet just as clearly Antonioni panders to those instincts with the endless sex and modeling, and 'cool. mod London' scenes. I'm sure he'd say that's the whole point; we're all that shallow underneath, but...
To be fair, many (most?) still think it's a masterpiece. And if more as an important piece of film history than entertainment, I agree it's worth seeing.