RogerEbert.com:

On paper, "Chennai Express" is as safe a bet as blockbusters get: a director with a string of hits to his name (Rohit Shetty) working with two huge movie stars (Shahrukh Khan and Deepika Padukone) whose previous collaboration yielded commercial and critical success (2007's "Om Shanti Om"). Add romance, comedy, action, and adventure, and you should be all set. Although Hollywood has seen would-be blockbusters falter this year, most of those were either prohibitively expensive to begin with, in unpopular genres, or marketed poorly (if at all). "Chennai Express" would seem to have none of those issues, and the only business-related question remaining is whether it's going to be a big hit, or an enormous hit.

Variety:

The humor is pretty one-note in Rohit Shetty’s comedy/romance/actioner “Chennai Express.” Cast as a sheltered 40-year-old suddenly thrust into multiple perilous situations, superstar Shah Rukh Khan repeatedly reacts with craven cowardice and flashes of false bravado; though played for laughs, his behavior comes across more as moral laxity than hilarious evasiveness. It doesn’t help that co-star Deepika Padukone is so likable as the rebellious daughter of a southern don, rendering the antipathy between their characters somewhat gratuitous. This North-South culture-clash laffer set an opening-day B.O. record in India with $1.1 million (beating the previous champ, “3 Idiots”), but elsewhere, its popularity may depend on audiences’ willingness to wait two hours for Khan to transform into his heroic self.

The New York Times: 

“Chennai Express” feels like a sumptuous meal with carefully chosen wine and tasty appetizers but a botched main course. Money and visual care have been lavished on this Bollywood action-comedy-romance and glossy stars engaged (Shah Rukh Khan and Deepika Padukone). But the movie chugs along for most of its 2 hours and 20 minutes searching for comedy and characters in a frantically overplotted story.