I've used nRelate on a couple of sites.
Best looking related content plugin I have found.
Many options for the display for example with and without thumbnails and excerps.
The option to include content from other websites (very useful).
Sometimes it just doesn't work.
Crappy relevancy of the related content. All other related content plugins I've tried has been better at actually matching posts with each other.
At the moment excerps isn't working.