Few book-to-film adaptations carry the same weight as the new movie version of writer Jack Kerouac's classic novel on the Beatnik era in the late 1940s, "On The Road." A generation defined itself against Kerouac's portrayal of America, the boundless electricity of his thirst for life, and poetic diction. Any movie based on that is inextricably tied to the book's enduring legacy and that potent moment in history.

Directed by Walter Salles ("The Motorcycle Diaries") "On The Road" is based on the years Kerouac spent travelling the United States in the late 1940s with his friend Neal Cassady, and several other figures who later became famous in their own right, such as William S. Burroughs and Allen Ginsberg.

Numerous directors - perhaps most famously Francis Ford Coppela - have attempted to transform the notoriously verbose prose of Kerouac into a film over the years, but none have ever made it far past the starting line, until now. As the first reviews spill out for the first-ever film version of Kerouac's famous ramble, many critics note the movie buckles under the pressure of fulfilling all those expectations; in trying to be everything to everyone who ever loved the novel, "On The Road," ends up feeling like a showroom model: all style, very little substance.

Overall, critics and audiences appear mixed in their reviews of "On The Road." The film has a current score of 47 percent on movie review site Rotten Tomatoes.

NPR understood the tune Salles was trying to sing with his adaptation, but thought that in the film's attempts to exemplify the time period it focused too much on characters indulging in drugs and sex, while never really fleshing out their traits a real human beings.

"What I wanted from 'On The Road' was something that would capture what people love about Beat literature. What I got was a movie that genuinely draws all its pleasures from people speaking painfully affected dialogue and doing lots of drugs and having lots of sex with each other. It's exactly the parts of life that are better to experience than they are to hear about. It's all just so much less interesting than you think it is when it's happening to you," wrote NPR.

"Whatever Kerouac's friends were like in real life and however he drew their analogues in the book, the people in the movie are not extremely fascinating. Similarly, there's a lot of sex in the film and a lot of it is supposed to be daring - look, three people! Look, two men! Look, they're doing it in the car! But as with the drugs, the handling of the sex is so glib that it's actually dull," NPR added.

The Associated Press liked the overall presentation, but thought the final product lacked true heart. Where the film really succeeds, it said, is in the surprising performances by its female characters, such as Kristen Steawrt, who at times overtake everything else in the picture.

"Walter Salles' 'On the Road' was made with noble intentions, finely-crafted filmmaking and handsome casting, but, alas, it does not burn, burn, burn... But this 'On the Road,' the first ever big-screen adaption of the Beat classic, doesn't pulse with the electric, mad rush of Kerouac's feverish phenomenon," Wrote the Associated Press.

"Salles has focused particularly on the carnality of Kerouac's tale, and it threatens to overtake the film. As Moriarty's first wife, Marylou, Kristen Stewart's slinky sensuality briefly dominates the movie, but her character is never developed beyond her sexy bohemia... The women of 'On the Road,' afterthoughts in the book, have more fire than the men. In the end, 'On the Road' remains paved over," the Associated Press concluded.

Variety, like NPR, found Salles' extreme focus on drugs and sex tedious at times, and thought the adaptation often hewed too closely to his source material at times, feeling removed, without an ounce of sincere emotion behind it.

"Music and wild dancing, unbridled sex, poetic streams of language: These are all good things, but in 'On the Road' they're staged with a resoundingly earnest, museum-piece diligence. And so it's a little hard to experience what the movie wants to give you, which is a contact high. Watching On the Road, there's a scrappy, lurching party going on - and also a fair amount of heartache - that we're not so much invited to as invited to stare at, as if this were a hallowed piece of cultural anthropology. And in a sense, that's just what it is... 'On the Road' is a curiously remote experience, all reason and no rhyme," wrote Variety in its review.

"The movie is about people reaching for sensations, experiences - a way of life - that hadn't been codified in the culture yet. But now, sixty years later, it has been, and 'On the Road,' while the movie may think it's showing you what things were like before, can't really imagine it."

"On The Road" hits theaters Dec. 21, and stars Sam Riley, Garrett Hedlund, Kristen Stewart, Amy Adams, Elisabeth Moss, Kirsten Dunst, Viggo Mortensen, Steve Buscemi, and Terrence Howard.