I've used Mantis in the past as a tool for field service personnel to report issues into QA; we did not want the SW Engineering bug tracking tool bombarded with field issues that could potentially be user error, configuration issues, etc. but also did not want to loose this valuable information that was coming in from the field; Mantis served that purpose well. Once field reported issues were verified QA Engineers will promote the Mantis issue into Bugzilla (our bug tracking tool) OR close the issue in Mantis with the possible resolution.
i have used it. its not complex one. but be careful because i was admin but could not find it as secure. because developers deleted all the reported bugs while they were not assigned to him. i mean i could not find privacy.