There are quite a few companies that support the CMIS standard. The participants in the standards process include Adobe Systems Incorporated, Alfresco, EMC, eXo, FatWire, HP, IBM, ISIS Papyrus, Liferay, Microsoft, Nuxeo, OpenText, Oracle, Newgen, OmniDocs, and SAP.
To get an idea of how widespread the CMIS standard is, we can have a look at the following products, clients, and libraries that support it:
CMS servers: Alfresco, Documentum, HP Interwoven, IBM Content Manager and FileNet, Lotus Quickr, Microsoft SharePoint, OpenText, and SAP
WCM systems: Magnolia, Liferay, Drupal, Hippo, TYPE3, and dotCMS
Blogging: Wordpress
Clients: Libre Office, Adobe Drive, Atlassian Confluence, SAP ECM Integration, Pentaho Data Integration, SugarCRM, Trac, Kofax, and Salesforce Files
SOA: Mule ESB and Spring Integration
Libraries: Apache Chemistry (which includes Java, Python, PHP, .NET, and Objective-C)
So we can see there is no doubt that the CMIS standard has been very well received and adopted.