Myself and a number of other MW enthusiasts host a couple of MW-based sites that make use of SMW.
A problem I came across when working with semantic relations was the lack of inverse properties, a way to define a name for an opposite relation (A is part of B, therefore, B contains A).
I have attempted to fix this issue with some minor changes to the SMW code. I am sure this is an obvious feature SMW will eventually have, as noted in the documentation.
Here is an example of my use of the changes I made:
http://www.nazuz.com/wiki/Heading_up_%28sailing%29 // this page contains the semantic data
http://www.nazuz.com/wiki/Reducing_heeling_%28sailing%29 // this page has limited semantic data, but much inverse data
http://www.nazuz.com/wiki/Property:Is_a_method_of // The inverse property of "Is a method of" is defined here. ( [[Inverse::Property:Can be achieved by]] )
I also had to setup http://www.nazuz.com/wiki/Property:Inverse . // Explanation is on the page itself.