I always use "--no-ff" when merging topic branches to master. The advantage is that you can keep individual commits on the topic branch readable and still get an overview of the changes between two releases with "git log --first-parent".

You can also see the set of changes in a topic merge easily with "git log ${merge}^2..${merge}^1" whereas if you use a fast-forward merge it's not at all obvious which sets of commits are related.