From the abstract of Arisholm et al's "The Impact of UML Documentation on Software Maintenance: An Experimental Evaluation" (IEEE Trans. Soft. Eng
, 32/6, June 2006):
Results show that, for complex tasks and past a certain learning curve, the availability of UML documentation may result in significant improvements in the functional orrectness of changes as well as the quality of their design. However, there does not seem to be any saving of time. For simpler tasks, the time needed to update the UML documentation may be substantial compared with the potential benefits, thus motivating the need for UML tools with better support for software maintenance.
Note that "...thus raising the question of whether UML is worth using at all" isn't presented as an option... ;-)