Some other criteria to watch out for:

• Teamwork: several people should be able to work in parallel
• Versioning: documentation artifacts need to be version-controlled. That’s quite easy for single files, but some modeling tools make versioning harder than it should be.

• Artifact generation: Sometimes you need beautiful or stakeholder-specific output (e.g. pdf, html). Some tools excel in this category, others fail miserably.

  Personal opinion (Gernot): Although several UML modeling tools promise to generate beautiful output, my experience tells a completely different story: Before you buy into one of these report generators, seriously battle-test it, especiall concerning the effort it takes to create and maintain the required output.

• Robustness: You don’t want to re-create diagrams or documentation because your boss made you use that (supposedly) cheap free-and-open-source documentation tool… I had especially bad experiences with immature graphic/UML editors.)
• Availability of know-how: For several of the more common tools it’s quite easy to find people with corresponding experience. Some tool vendors provide excellent support, also in methodical questions.