One thing that helps Shamitabh in making it a decent watch is a new story line. The concept itself is not yet tried in Bollywood. The trailer pretty much speaks about the story yet watching it unfolding on screen can be interesting.
The second great thing about Shamitabh is the performances of Amitabh and Dhanush. Dhanush just nails it again and manages to get noticed even when he shares the frame with Indian legend Amitabh Bachchan. Kudos!
On the flip side you might have wanted the screenplay of the movie to be more tight as there are few dull moments in the movie which at times hamper the narrative.
Nevertheless, Shamitabh is a one time watch owing to fresh new story and decent performances by its lead protagonists