Jailer succeeds due to three things - a good storyline, the cast and the music. Among the cast, Rajini - at 72 - has played an age appropriate role and has done a wonderful job.
These positives offset the weak screenplay. The Director lacks class and has a silly sense of humor (Beast, etc). In the hands of a more able director, the movie would have been even better.
But Rajiniโs charisma is still strong and allows the movie to rise above these weaknesses - making it more entertaining than most movies.