The answer to this question is often very telling about a given candidate. It shows
1) whether or not they know what they’re talking about in terms of development, and
2) it really illustrates the maturity of the individual (a common theme among my questions).
My main goal here is to get them to show me pros and cons for each. If I just get the “many eyes” regurgitation then I’ll know he’s read Slashdot and not much else. And if I just get the “people in China can put anything in the kernel” routine then I’ll know he’s not so good at looking at the complete picture.
The ideal answer involves the size of the project, how many developers are working on it (and what their backgrounds are), and most importantly — quality control. In short, there’s no way to tell the quality of a project simply by knowing that it’s either open-source or proprietary. There are many examples of horribly insecure applications that came from both camps.