Group Sum Sudoku (Daily Sudoku League #44)

When the Indian team came back after attending World Puzzle Championship in the year 2005 which was held in Eger, Hungary, we were very much impressed with the puzzle creation skills of Hungarian puzzle creators. We got inspired to start learning the art of puzzle creation. Some of us in India formed a yahoo group and started creating one puzzle on the weekly basis and publish it in the yahoo group and others on the group will solve it and give feedback so that everyone can learn the art of puzzle creation. This is the puzzle I created in March 2007 being part of that yahoo group. This Sudoku type appeared in Indian National Sudoku Championship 2007. After that, it also appeared in World Sudoku Championship 2008 at Goa India. This was the most feared Sudoku type of any of the championships in which it appeared. This puzzle type appeared again recently in Times Sudoku Championship 2014 Mumbai finals.
The feedback which I received for this particular puzzle was quite good, so I am publishing this Gropu Sum Sudoku puzzle as part of The League of Extraordinary Ladies and Gentlemen as my 44th contribution.