Do you need to be found?
If you want to become more visible on search engines and get more traffic to your site you should definitely consider Schema.org. On the other hand if you have a very popular website that people navigate to directly, then Schema.org might not be as valuable for you.
Do you want to make your data visible?
If you have data you want to make accessible to others, use Schema.org, but be aware that this will also make it easier for others to copy your data and use it for malicious purposes.
Do you want to provide a better search experience on search engines?
If you do want to provide better information to search engines so users do not need to navigate around on your site to find what he or she really is looking for then consider Schema.org.
If you have advertisement banners on your site Schema.org is likely to reduce the number of page views on your site since users will be able to filter and make more precise searches before they land on your site. In some cases users might find the information they are looking for just by viewing the search results on Google, and so there is no need to visit your page at all.
Do you have the time?
Although we have made it extremely easy for you to add Schema.org to your site it does usually take a few hours to enable it for your site and to set up type mappings etc. You are also likely to spend some time improving mappings etc. as part of normal SEO enhancements or as Schema.org evolve.