Indiewire:

But Riley's true counterpoint in the story comes from Stewart's achievement as the giddy, pleasure-seeking Marylou, a credible performance made particularly noteworthy for her current fame in the "Twilight" franchise; frequently going nude, speaking up and dominating most of her scenes, she buries her movie stardom with this refreshingly non-commercial gig.

THR:

Stewart, selected for Marylou five years ago on the basis of her striking debut in Into the Wild, is perfect in the role, takes off her clothes more than once and nearly always seems to be breaking a sweat, which kicks the sexiness quotient up high.

Variety:

The other actors hit their notes effectively, particularly Mortensen and Sturridge as the respective alter egos of William S. Burroughs and Allen Ginsberg; and Stewart and Dunst, whose warm, emotionally accessible turns lend Marylou and Camille more flesh and character than they had on the page.

Vulture:

Certainly, there's nothing regrettable about Stewart's performance here: It reestablishes the promising character actress last seen in Into the Wild and held captive as Twilight's leading lady for years. Though Stewart delivered a great turn as Joan Jett in The Runaways, she had been field-testing that performance in real life for so long before the movie came out — donning a black mullet, rocker sneer, and Minor Threat T-shirt on the red carpet and at Comic-Con — that audiences gave her little credit for it, assuming the character was not far removed from Stewart herself. Mary Lou is a different kind of thing entirely, though: Stewart makes eye contact instead of dropping it, and the lip-biting tics that the actress brought to Bella are all banished. Stewart's honey-haired Mary Lou knows what she wants and goes after it, and Stewart gets to show off a confidently sensual side of herself, dancing with abandon and seducing at will.

HitFix:

Kristen Stewart plays Marylou, one of the many women who exist in Dean's orbit, drawn to his life-force and repelled by his inability to stay in one place, and it's good work from her, further indication that as soon as she puts the "Twilight" series in her rearview mirror, she's got a promising career ahead of her. There is something simultaneously innocent and carnal about Marylou. There's no guile to her, and she is very clear about what she wants in every scene, very direct in the way she obtains it.

Ain't It Cool News:

Kristen Stewart plays Marylou… she some of the glue that bonds Sal to Dean. Stewart plays Marylou a little sad, but mostly as a base layer. Her mood is typically energetic, happy and a little minxy. Okay, more than a little minxy, as you can tell. It’s a brave performance for her. Yes, there’s the nudity, but there’s also the emotional ambiguity to her character. She can be in love with Dean Moriarty and also hate him. If she played it wrong then Marylou would just come off as a slut with stars in her eyes, but she injects the right amount of vulnerability to make her sympathetic, but not undercut the confidence of Marylou as a character.

The Film Stage:

Kristen Stewart should gain some non-Twilight followers thanks to her very physical, interesting performance. Stewart’s Marylou and Hedlund’s Dean have palpable chemistry, providing a sexiness to the proceedings that’s rather unexpected and one of the more engaging recurrences here.

Scene Creek:

I’m sorry, Twilight completely undervalues Kristen Stewart’s acting capabilities. I don’t know why they had cast her as a virginous vampire lover. She definitely plays a better cracked out lover than anything Kirsten Dunst can do. I mean, it takes a completely different woman to sit naked between two dudes and jack them off simultaneously in a car.

Filmoria:

Stewart is also fantastic here, giving the type of performances she thrives with, which are often overshadowed by her Twilight alter-ego. In fact, she’s probably the strongest out of the three; her Marylou rings of her role as Em in Adventureland which is perhaps her most established and confident screen-turn in a major release. One has always defended Stewart’s abilities and many-a-critic have been biting their lips after watching her in On The Road today.

The Independent:

The most surprising turn is Kristen Stewart's as Marylou/LuAnne Henderson. The camera absolutely loves her, despite her awkward gait. She thrives as the promiscuous fun-loving girl, who literally gets in between Sal and Dean. The scenes with her are electric.

Premiere (France)

The outrageous conduct of the Twilight star (and let’s not forget Welcome to the Rileys) not only makes her affecting, but it also shows a boldness that is rare in actresses of her generation.