We use Scrum project management methodology that works great if you know how to do it right. I recommend the book Scrum and XP from the Trenches by Henrik Kniberg, we have learned a lot from it.

One of the best things in software development using Scrum is high return on investment (ROI) for the customer and smooth work for the development team without big bottlenecks. Work becomes pleasure when is managed in Agile fashion.

If you use old-fashioned Waterfall principles for your project (that are still great for manufacturing, but don't work in software business), you can implement 10 features from 12 planned (for example) if you meet risks, planning issues or changing business needs.

If you use Scrum, you can implement the same 10 features from 12 planned because of the same reasons, but those 10 features may differ a lot from what Waterfall would offer.

With your 10 features implemented using Agile techniques you will probably get more benefits, because they are what you need today, not what you signed off 4 months ago. This is a big difference. You pay only for feautures you need when you have more information, not for those that exist on the contract.

And this is the rule: in every project about 50% of all features are less important and means lower ROI. Furthermore, only about 20% of planned features are really important for benefiting in less time, and the rest of them can be reprioritized and implemented in the near future.