Open source means that it is not compiled. That the source code is available to you, and that means OPEN. You need the source code (or compiled version) of software (be it only an application or an operating system) in order to advance it or build on it. You can also still use it without modifying it. The problem is that since it’s “free” there might not be any real support for end users. You will have to be quite savvy with internet research (know how to get results from search engines) since most or all knowledge will have to be obtained through forums, etc. Take Linux, open source, endless information on the internet via forums and wikis.
So, open source software can be great for developers and people who know what they are doing. But less desirable for soccer moms. The internet, however, with its instant gratification can make open source the future of software development.