I have to admit I have always found it difficult to watch an Antonioni film from start to finish at the first try, and even for this one, I ended up watching it in three parts on repeated occasions. In the end, I realised perhaps it was better that way, because it forced me to stop thinking in the usual terms of plot and just enjoy the scenes one by one.
The first part seems a lot more fragmentary, which is not a bad thing, it just requires more of an effort to follow. When it gets to the desert scenes, all efforts are repaid in full. The stunning cinematography is only a part of it, what really makes it all unforgettable is how the landscape is made into an overwhelming presence, the silence and vastness of it, the sense of sadness and freedom, the way it fuses and contrasts with the two young characters. The desert is dead, but at the same time it feels less distant and alien than the urban scenes in the first part. The dance between the airplane flying over and the girl's car makes for a series of great shots. The love scenes in the desert are simply beautiful, it is hard to imagine this kind of approach from a film of our times. There is of course something very stylish and studied about them, but at the same time they manage to express a sense of natural, spontaneous innocence that is very rare these days. It all feels loose and unscripted (thanks also to the understated acting), but that is the result of a maniacal attention to detail and form, which comes to its climax in the series of explosions at the end, a really mesmerising spectacle. It just leaves you in awe.
I don't really care for some the usual objections: boring - well, yes, it is, if you want all films to follow a classic plot development and be packed with action twists, but then if all cinema was like that, that would be truly dull and sad; pretentious - maybe, but when that kind of ambitiousness is coupled with actual skills, depth, and style, pretentious is a compliment. The "political" criticisms make the least sense to me, I don't see the point of approaching a film like this with ideological blinders or worse, patriotic requirements. It just defeats the purpose. Perhaps it's true that, like a previous commenter remarked, Antonioni viewed these young 'hippies' and the politics of protests and riots with the police with the fascination of a foreigner, but I think that adds something rather than detracting from the film. It's not true that hippies did not exist in Italy at the time (think of the '68 protests, like in France), although they were obviously different from the American counterpart and in some ways even more militant. But his interest in this film was not narrowly political. The events seem more like a pretext for a film whose appeal has a universal, timeless quality.
A special mention for the fantastic soundtrack. Amongst other things, this film, along with Easy Rider, is probably one of the main earliest precursors of the contemporary 'artsy' music video as well as the concept of a film soundtrack that would stand on its own, but unlike the former, it uses music in a much more subtle way, blending it with the landscape rather than the action.
If you want traditional narrative in a film, then don't bother. If you want to be stunned, be patient and you won't regret it.

The only thing that "An Inconvenient Truth" proves is that Al Gore is still an idiot. These "unchallenged" experts are unchallenged because a response to their inane hypotheses is generally beneath real science. This is mostly false science folks. The greatest source of greenhouse gases - CO2 - is people, we exhale it and unless you're willing to start sacrificing your brethren to save the world, there's not a darn thing to be done. We've heard how the world was going to end as the result of man for more than 50 years. Fools publish a time line for their doomsday and when the time passes, nothing has happened. "An Inconvenient Truth" is just another vehicle with which a disingenuous faction of American society can peddle their poop.
And as to Al leaving the tobacco business because of his sister's death from cancer, that is a load too. Al couldn't run his farm any better than he could run the country. He was losing money on the operation because he didn't care to farm when he could make more $ on speaking tours. The only global warming that is unchallenged is the hot air produced by this gasbag!