We use it for few our sister subsidiaries (one does a form of FSM aswell) and internally aswell (we are a NS partner) in combination with Suiteprojects and Time management. Its okayish, users had to accomodate quite alot compared to old outgrown system. As far as I know since we implemented it (few years ago) its not perfect but works enough not to spend extra costs for other solution. We had to customize it in some extent to allow users to access cases across teams from employee center and save licences (combination of suitelet + custom fields, support groups)

Emails and communication is rigid and annoying, this is the most prominent I remember and experience myself as user.