"Rockstar is easily Imtiaz Ali's best. This is how a love story should be told. Soul stirring. Still assimilating," Kashyap wrote on microblogging site Twitter.
"'Rockstar' is unlike most films...no cacophony of sounds...quiet...no unnecessary background to propel it forward...Ranbir is just awesome. It is not 'Jab We Met'...its come from somewhere with in Imtiaz. It's his labour of love," he added.
Released Friday, "Rockstar" chronicles the journey of an aspiring singer Janardhan Jakhar, who wants to make it big in the music world and what he endures to earn fame. It introduces two new faces - Pakistani-American model Nargi Fakhri and Brazilian model Moufid Aziz.
Kashyap, known for making unconventional films like "Black Friday" and "Dev D", also calls it a "coming of age film".
"Such an unexpected film.. does not take any easy route.. does not become a clap trap or a crowd pleaser.. It's just so heartfelt...," Anurag wrote.
"'Rockstar' is not so much about rock as it is an individual's story..it's another kind of coming of age film...just can't get it out of my head," he added.