Gojko Adzic – Delivering useful software:

"If an iteration produces software that can really be used every day, then ship it and make the clients use it. It is definitely useful to get feedback early, but having something go live, even if it causes rework or throwing away parts of software later, is much much more valuable than software that is only tested. It has more value for the customer, since they are getting a part of their problem solved earlier. It also has more value for the development of the whole project, since it brings real, authoritative feedback from everyday use."