Shankar and Vikram have done a fantastic job. Director Shankar is turning into a professional high-quality perfectionist. Actor Vikram, charged with a National award for Best Actor (2004) is on a roll. His performance is classy and comparable to many great actors of Tamil cinema.
Anniyan is definitely worth watching at least once. Forget about the critics who list past movies from which various aspects of Anniyan (like concept, theme, stunts, music) was inspired (or copied). It is easier to make the original than to make a copy as the original serves as a hard benchmark. It is hard to replicate something while maintaining audience interest. Shankar does this job to a more-than-satisfying extent.
Besides, when great directors like Steven Spielberg copy past movies (War of the worlds) with the confidence of providing the viewers something worthwhile to watch, I don't see anything wrong in Shankar's Anniyan. Yet, Anniyan is NOT a complete copy and it is definitely MORE than the sum of its parts.
Anniyan has stunts that were never before seen in any Tamil movie. Shankar has provided the opportunity to the (uneducated mass of) Tamil audience of watching world-class stunts in a movie that they can understand (as against most Hollywood flicks that have the action but make no sense to them). So what, if it has been copied? Matrix Reloaded being largely mystifying and unexplained, even to most native-English speakers, could not justify to the (masses of the) Tamil audience as to why all the action exists!!??. But that is not the case with Anniyan.
To sum it all up, Anniyan is what a movie should really be. A classy entertainer. Good songs, great cinematography, wonderful (although sometimes unbelievable) stunts, great acting performance, nice comedy and classic direction. Watch the movie without prejudice and be entertained.