Advantages of open source: quickly working around problems
Yesterday was fun. We use SBCL (Common Lisp) on a project I work on, and my customer hit a limit of SBCL not using all the memory on one of their new servers. A quick email to a SBCL developer, and we were patched and running very quickly (thanks Nikodemus!)
A decision to use any open source project is usually based on how "vibrant" and active the community is surrounding an open source project. This is easy to determine developer activity by release history and user base by newsgroups + wikis.
I definitely have my favorite commercial developers (plugs to Franz Lisp, Jetbrains/IntelliJ, OmniGroup/OmniGraffle, and TextMate) and my decision to purchase and use commercial products is based strongly on support, patches, and new versions.