First things first - Ranbir nailed it as Baba...there is no second ways about it. He looks, walks, talks like Sanju and is the best thing in Sanju. The second best thing in Sanju is Vicky Kaushal - had they written the part better for the second half, maybe it would have been the best part! Close 3rd is Manisha Koirala as Nargis...amazing Grace and calmness...beautiful potrayal without hamming at all.
Everything else is a drag...you can see that maybe Hirani is completely out of his comfort zones with no practical jokes and common man dialaogues and hook line (jaadu ki jhappi, wrong number hai, all is well) to play with. The background score is super amateurish...the typical 3 idiots, PK type of mouth organish pieces...doesn't work in a biopic...horribly disappointed.
The film might work - would work- simply on curiosity. Is high on emotion - purely coz Ranbir, Vicky and Manisha understood the premise and lent their maturity...but script didn't work for me.
All in all...good watch..but like someone else said in review - you don't remember anything coming out of the movie.