Open source has many, many advantages. After working for Mozilla for a year, I almost can't imagine how it is possible to work on a closed source project. Having the involvement of a wider community, being able to search the web for our code, our bugs, documentation, blogs giving insight into the code, not worrying about secrets, and so forth are truly wonderful. Contributing to an open source project is also the best way to learn about software engineering and any specific domain of it. If you are a student, or are looking for work, or looking to improve your software skills in any way, then there is absolutely nothing better you can do for yourself than to find an open source project and get stuck in (plug time - anyone interested in graphics in web browsers should get in touch!). It is probably my biggest regret about university that I didn't get involved with some open source projects, and instead worked on my own projects.
Anyway, all this is in the past. As of the last few weeks I LOVE open source and I am now truly a believer. The reason is that I acquired some new hardware, in particular a Raspberry Pi and a Samsung Chromebook. Both of these have ARM processors, which although found in pretty much every phone and tablet, are pretty much a minority interest in terms of 'real' computing. First off I have been amazed at the quantity and quality of open source software specifically aimed at such devices. There are no closed source equivalents and, due to economics I suppose, there never will be for niche areas like this.
Secondly, and here is the amazing bit, where the software I want for the platform I want doesn't exist, I can just compile it! This is so simple, yet so powerful. Even for niche areas you can usually get things like an OS and a browser, but what about all the other bits and pieces you want? For example, I use Sublime Text 2 as my main editor, it is a lovely piece of software and I use it in Windows, Linux, and on Mac. But, it is not open source and there are no binaries for Linux/ARM, so I cannot use it. But SciTE is also a lovely text editor and IS open source, so I can just compile it and use it anywhere I want. That is amazing! No really, we take this a bit for granted, but in terms of encouraging innovation, open source is miles ahead due to this very simple fact.