It is important to understand whether or not your project is on track to meet your planned milestones and final delivery date. This should not be a gut feeling, and should be based on your actual progress to date compared to your planned progress to date. It is a simple calculation derived by:

Calculating the total Planned Effort of all user stories in the project, then
Calculating the total Percent Complete for all user stories (entered by users as they update their tasks).
Multiplying the total Planned Effort by total Percent Complete to give the Completed Effort.
The Completed Effort result is then divided by the total Planned Effort and multiplied by 100.

For example:

User Story	Planned Tasks	Percentage Complete	Completed Effort
User Story A	80 tasks	90%	72 tasks
User Story B	20 tasks	90%	18 tasks
User Story C	40 tasks	50%	20 tasks
User Story D	20 tasks	40%	8 tasks
Totals	160 tasks	-	118 tasks

Therefore 118 tasks / 160 tasks x 100 = 73.75% Project Percent Complete

Note for Project Manager:

Each week you should be measuring your project % complete and comparing that to your project % goal for that point in time.
If you find that your project is a lower % complete than planned you should then drill down into the user story or task level to understand where you're behind, why you're behind, and what it will take to catch up (e.g. adjust resources, adjust scope, extend time to that area, etc...)