Quoting from the Schema.org Web site:

“Schema.org is a joint effort, in the spirit of sitemaps.org, to improve the web by creating a structured data markup schema supported by major search engines. On-page markup helps search engines understand the information on web pages and provide richer search results. A shared markup vocabulary makes easier for webmasters to decide on a markup schema and get the maximum benefit for their efforts. Search engines want to make it easier for people to find relevant information on the web. Markup can also enable new tools and applications that make use of the structure.”

Google and other major search engines are cooperating in this effort.

Accordingly, it’s widely agreed that use of Schema.org markup is beneficial to search-engine optimization.

Estimates of the incidence of use of Schema.org markup range from 1% to 31% of all Web sites, (So nobody really knows.)

Schema.org tool developers Schema App have published results of a survey on the experience of marketers using Schema.org markup.

Highlights:

Their sample population included only 75 individuals, which is pretty thin. Keep that in mind.

40% of respondents work in companies with fewer than 5 employees, and 45% identify themselves as SEO Specialists.

About 92% of these folks said they update their markup (not necessarily often) as Schema.org versions advance.

Biggest challenges in using Schema.org markup that survey respondents mentioned were:

1. Demonstrating value in terms of results (45%)
2. Keeping up with Google changes (42%)
3. Developing a strategy as to what to mark up (40%)
4. Scaling for large Web sites (37%)
5. Understanding Schema.org markup vocabulary (24%)

Although there are a few tools available to help with Schema.org markup, 40% of survey respondents said that do the process manually.

FYI (not endorsements):

A tool I use is Google’s Structured Data Markup Helper (link below), which is helpful up to a point but by no means a panacea.

And here are some guidelines from Schema App for measuring results of Schema.org markup.