When migrating from our previous bug tracker (ancient installation of Bugzilla) to MantisBT this summer, we considered Redmine among other alternatives (MantisBT, Bugzilla, JIRA). All have their own pros and cons, so we decided to use Mantis since it is simple and does all what we needed, and does that (almost) in a way we like (at least, get accustomed to). Mantis got wide use in our projects last years, and we preferred to work with the same system in this project, too.
Mantis as well as Redmine provides version management (see Roadmap and Change log), issues relationships, has plug-ins for Wiki and version control integration, and even for project management facilities such as time tracking. That is, the functionality it provides is quite comparable to that of Redmine. The difference is mostly in presentation and level of integration between components, and I agree that Redmine looks better in this aspect.
We are using Redmine internally, and personally I like it for the nice way it presents the data, and especially mail notifications. However, when trying to use it on more-or-less real cases, I found its behavior unpleasant. Not sure to recall all issues, but at least several times I have lost some text I tried to input in the issues because when I tried to save it, Redmine just logged me off. Another issue I have is that trying to connect to the current OCCT SVN repository, all what I get is message 404 "The entry or revision was not found in the repository". The same happens when I click on some user names (including my own) -- instead of viewing user profile, I see just an error message.
There are also some conceptual problems to consider if we adopt Redmine. First, it is tailored to running complex projects rather than plain bug tracking. What we have in OCCT is mostly bug tracking, and Redmine's emphasis on project management would likely be useless.
Next, we want to keep all the issues we have on OCCT in one tracker system, but at the same time we need to separate issues created by commercial customers, and restrict access to issues on per-customer basis (for confidentiality reasons). This is not usual practice, and neither Mantis nor Redmine provide such possibility readily. Now the solution we have found is that we mapped customers to (sub)projects in Mantis. This approach also renders 'normal' project management features useless.
Finally, we need to compare tools for Git integration and code review provided by Redmine and Gitorious, to see if one is much better than another for our needs. If we end up with using Gitorious, Git integration capabilities of Redmine will become out of interest.
That is, I do not exclude that one day we decide to switch to Redmine, but this move should be carefully analyzed before. The idea of using one tool is quite attractive, and we definitely will pay more attention to Redmine.