1) Reliability: Tests have shown that FOSS products have an edge in reliability. E.g. 

• IBM studies found GNU/Linux highly reliable. IBM ran a series of extremely stressful tests for 30 and 60 days, and found that the Linux kernel and other core OS components -- including libraries, device drivers, file systems, networking, IPC, and memory management -- operated consistently and completed all the expected durations of runs with zero critical system failures. Source: http://www.ibm.com/
• When problems are found they are often fixed quickly within the community. Actually the people who spearhead in fixing of these problems are usually not the original developers (FOSS). This is possible because the source code is open to all.

2) Performance: Comparing GNU/Linux and Microsoft Windows performance on equivalent hardware has a history of contentious claims and different results based on different assumptions. FOSS has at least shown that it's often competitive, and in many circumstances it beats the competition. E.g.

• Performance tests for file servers found that Linux with Samba significantly outperformed Windows 2000. Samba surpassed Windows 2000 by about 100% and can handle 4 times as many clients. Indeed the new release Samba 3 extends the lead over Windows 2003. Source: http://www.vnunet.com/
• Tests have also shown that the FOSS program MySQL was quite comparable to the proprietary Oracle database program, and the pair outperformed other proprietary programs.

 

3) Scalability: Many people mean by “scalability” to answer the question, “can you use the same software system for both small and large projects?” (ability to grow the system as needs demand without costly modifications ). FOSS is unbeatable in this area. GNU/Linux for example works on PDAs, obsolete hardware (so you needn't throw the hardware away), common modern PC hardware, over a dozen different chipsets (not just Intel x86s), mainframes, and a number of supercomputers.

4) Security: Quantitatively measuring security is very difficult. However, there have been a few attempts to do so, and they suggest that FOSS is often superior to proprietary systems, at least in some cases. E.g

• “hacker insurance” costs 5-15% more if Windows is used instead of UNIX or GNU/Linux for Internet operation. Also, there are about 60,000 viruses known for Windows, 40 or so for the Macintosh, about 5 for commercial Unix versions, and perhaps 40 for Linux.  
• Computer viruses are overwhelmingly more prevalent on Windows than any other system. Virus infection has been a major cost to users of Microsoft Windows. The LoveLetter virus alone is estimated to have cost $960 million in direct costs and $7.7 billion in lost productivity, and the anti-virus software industry sales total nearly $1 billion annually. There are about 60,000 viruses known for Windows, 40 or so for the Macintosh, about 5 for commercial Unix versions, and perhaps 40 for Linux. Most of the Windows viruses are not important, but many hundreds have caused widespread damage. Two or three of the Macintosh viruses were widespread enough to be of importance. None of the Unix or Linux viruses became widespread - most were confined to the laboratory.  
• According to a June 2004 study by Sandvine, 80% of all spam is sent by infected Windows PCs. 80% of all spam comes from computers contaminated with Trojan horse infections, according to a study by network management firm Sandvine. Trojans and worms with backdoor components turn infected PCs into drones in vast networks of compromised zombie PCs.Source: http://www.theregister.co.uk/

5) Flexibility and Freedom : Open source products may be tailoered to your needs. Open source products also do not tie you to your vendor, the software consumer is given control over the operations around the software.