Need to create a report where columns denote months, rows denote completed tasks across month.

Here is what you have to to:
1- Create a "task report" and make sure it uses a "process model" as context. and choose your process model.
2- Create the required columns (ie. User, Jan, Feb, ..., Dec, Total)
3- On the "Reports Option" window, check the "Group By" option next to the "User" column.
4- Click on the "Jan" column to edit its properties and do the following:
----- a. Put this in the definition field =if(month(task_completion_time())=1,1,0)
----- b. Choose "Number" as Formatting.
----- c. Choose "Sum" as Column Aggregation.
5- Do the same for each month but make sure change the condition to mach the month. For example for June you should use this as definition =if(month(task_completion_time())=6,1,0)
6- for the "Total" column, just put 1 in the Definetion field. And do the same as the points "b" and "c" from above.
7- Now switch to the "Filters" tab. And Check the "Completed" option.
8- That's it :)

Hey thanks a ton! worked like a charm!
Going further, I tried (without success :( ) to have a report for a user which will have months on top and "On Time tasks", "Late Tasks" and "Total Tasks" on the left. I tried returing values on 2 conditions (1st to check the month, 2nd to check the OnTime/Late boolean) However summation can only happen on "1" and count cannot be used. Any pointers will be helpful. Thanks

forget the above post, got it ... returned 1 for both true and false, and grouped them... Thanks for help anyways.