Open source, pros and cons?

I administer a network for a medium sized non-profit organization. We still use quite a few legacy software programs because we can’t afford the upgrades or licensing.

I’ve done a lot of research and testing on open source options for our productivity software. I’ve had some successes and had some failures.

I was wondering what experiences others have had. Are there any products you particularly like/dislike? I’m very interested in possible options to replace Outlook (that will talk to my Exchange Server) and Photoshop.

Thanks