Enough has been blogged to date about the main benefits of Open Source: Democratization, Innovation, etc. This are of course key. However, as a recent convert from the dark side (a.k.a Enterprise Sales) what strikes me the most is the impact on customer relationships.
Having been in the industry for 17 years -- the last 10 working for vendors -- the notion of "transparency" that comes with applying an OSS style to one's business model makes for more than a slick marketing slide. It permeates the software company's culture and drastically transforms relationships across-the-board. Hyperbole? Perhaps, but the next time you feel like something is being withheld by a proprietary vendor during a sales cycle, it's probably because you're right...
When prospects can instead download both your product as well as its source code freely on the Internet, that changes things. The whole discussion becomes one of making projects succeed, rather than finding the hidden gotchas in locked-down software. Who wants to start of a relationship with so much (almost expected) negativity?
Approaching things in an open source manner has impact far beyond the Engineering labs of a company. It's not simply a development methodology. Nor is it solely a fundamental shift in business models. While both are true, the real win for customers is the ability to enter into relationships with someone who wants to see them succeed with their software.