hi questlers... it seems there's a misinterpretation to the term open source, open source is a concept, it applies to small scripts (e.g. feedback form) to the big ones (e.g. cms, crm)..
"Open source is a set of principles and practices on how to write software" -Wikipedia

so it's not really a matter of haveing a support or not (it's a good thing to have a support or even comments, but not necessarily associated with open source as a concept) we've been using open source (i.e. LAMP) for a long time, and we used a "corporate" technology (i.e. asp.net) for couple of times... as said before, every body is thinking from a developer point of view, but when we wanted to create application to sell as software, we found that asp.net is better to protect our code and our hard work, no company who sells softwares for a living would like their hard work to be "open" to every one to take from unless they had to.. I know there is ways to encrypt PHP which we used before (e.g. Zend encryption) but it's still not as efficient as asp.net... this subject got me heated up, I should probably write a blog about it on the weekend and publish a copy here :)

I'd agree with Mamod, that is to check how active and how large the development community behind a specific project before you use its code.

Of course I believe in open source, and the most considered disadvantage is the development process, where at some point the project may die especially if there is no funds. the result is an out dated software, no new features no support and no patches.

Other than the above, everything else is an advantage :)

My advice if you want to use an open source is to make a research first, make sure it's an active project, make sure it has a support forum and a strong community, just like wordpress or any other strong open sourced.