
Rajni is Boss. No doubt about it.
But first half of Sivaji: The Boss disappointed. A sad heroine character and some attempts to be funny that fall flat (despite best efforts from Vivek). But the lion has to roar, and he does. There's quite an amount of fun in the second half.
If I were to pick the best thing about the film, it would be the impressive screen presence of the villain (I don't know the actor's name).

Anyone who loves the superstar would have already watched Sivaji, but amidst all the hype around the film, let us not forget a long line of films that made the superstar -- Ranga, Thangamakan, Moonru mugham, Padikkatthavan, Maaveeran, Velaikkaaran, Mappilai, Annamalai, Mannan, Uzhaippali, Yejamaan, Muthu, Basha, Arunachalam, Padayappa..
And when Sivaji: The Boss is getting so much acceptance in the metros and multiplexes, when it is getting released simultaneously in as many as 72 centres in Kerala (the highest for a Malayalam film has been 60), one must remember that less than ten years back thearte owners in Kerala were refusing to play Padaiyappa because they didn't want the "uncultured" Tamil audiences to spoil their halls.
It is intereresting to see how the amount of money involved has made all the difference. What else could it be?