Reasoning recently did a review of Apache's code and found that the defects per thousand lines of code were on par with commercial products in the same stage of development. Although the number of defects were slightly higher (by .02 per thousand lines of code,) Reasoning determined that the end result may turn out to be a better product due to the nature of open source projects—implying that bugs found are more likely to be addressed quickly by the public, since anyone has the ability to track down and repair the bugs.
"The open-source code seems to start at the same defect rate for early commercial code as well," Jeff Klagenberg, director of project management, said in an interview. "Over time, it can gain higher levels of quality. That appears to be because of the natural inspection process inherent in open source."
I hate to see blanket statements like this. I do think when an open source project has financial backing by a large company, that this probably holds true, but let's face it, there's only so much energy someone will put into a open source project before needing some kind of monetary return.
Programming can eat up lots of your time and at some point it comes down to either paying your bills or having free time—and without seeing some kind of reward for your work, you'll lose interest in the project. This isn't to say that monetary rewards will always keep your interest, but many unfunded open source projects never get beyond an initial release point.