<p>What are the pros and cons of adopting Open Source software for an organisation? Is there anybody out there who has done this and how well has it been working out with some examples of the softwares they adopted and how it has been in use?</p>

<p>Usually contributions come because people do it as a hobby, then how can we make sure that there will be continued support for it? IMHO, in case of proprietary software there is an incentive for the organisation (money), and they will keep hiring people to keep it under development as long as the software is profitable. Correct me if I am wrong. What are the arguments I might expect from a Manager who might oppose the suggestion to use Open Source softwares?</p>

The term "Open Source" only describes a licensing model.  Strictly speaking, the only pro that you are guaranteed to have are the freedoms given by the license, and there are no cons that you are guaranteed to have.
There are many Open Source products that are also commercial, created, maintained, and supported by a company for a profit.  There are also many Open Source products that are maintained by volunteers but also supported commercially.  For example, if you buy Red Hat Enterprise Linux, then Red Hat will support you on all of the products that come with it, even the ones that are maintained by volunteers.
As for how to be sure that there will be continued support, you can't.  Not with Open Source, not with proprietary software, not with anything.  With Open Source, if the community is large enough, you can be reasonably confident that the community will continue to maintain it (maybe under a new name) even if the current maintainers abandon it, and you have the option of maintaining it yourself or hiring someone else to do it.  Maintaining it yourself may not be an attractive option, but it can be a life saver in a pinch.
With proprietary software, if the author decides to stop maintaining it, you are just plain out of luck.  Consider, for example, the thousands of users of Visual Basic 6.