I recently had to use two very comparable products, one open-source and the other closed-source. While the closed-source product had more verbose documentation, but I actually managed to get the open-source product running and not the closed-source product.
More importantly, the API of the closed-source product, while very similar and accomplished the *identical* task, felt awkard. I guess that bouncing ideas off users and listening to what they have to say makes a real difference at the end.
Although clearly at a commercial disadvantage, an open-source project has a structural advantage at creating a better product. Of course for really large products where the combined efforts of dozens of programmers are needed for prolonged periods, closed-source remains a valid alternative.