http://blog.cloudbees.com/2011/12/jenkins-community-survey-results-82.html

The results from the first Jenkins Community Survey are in! 624 people took the time to share how they use Jenkins and tell us what capabilities matter most to them. - 82% of respondents said Jenkins is mission critical to their development process - 71% of survey respondents use Jenkins as their only CI tool • Jenkins is useful for companies of any size, but 76% work for organizations with > 10 - 80% of respondents have 3 or fewer masters and 10 or fewer build agents

More detail in the report...