I use it for the games shop on my website. It allows me to create a design for the product page and then use Jumi in a seperate article for each game and I just "fill in the blanks". This way if I want to change the way the product page looks, or its layout, I simple edit one block of html/code and every single product will be updated without the need to edit the product pages individually.
I don't see how I could ever live without this extension, now that I am aware of it.
The developer also has a dedicated thread on the Joomla forums and is superbly active, offering triple-A support that many companies could only dream of.
If you use this extension, I urge you to make a donation.