Burn-Down chart is one of the most useful project progress metrics. It shows how many features (user stories) where accomplished during previous sprints (iterations) and clearly indicates progress. It came from SCRUM and widely used in agile projects. However, to achieve good metric with Burn-Down chart, you should know some tips.
The major one is to break project into quite comparable features. I mean comparable by effort.
For example, you have 10 features. One of them will take 20 days, other 4 will take 10 days, and the rest 5 will take 2 days each. Do you think Burn-Down will be a good progress indicator in this case? No for sure. For example, you can implement all small features during the first iteration, and spend all of you time on big fat feature implementation in iteration #2. So Burn-Down will show great speed decreasing, but this is likely not true in this case.
So it is better to split the big feature into several smaller. Yes, this is a kind of an art, but you can track progress more precisely in this case.