I participated this summer and I would agree - it’s an amazing experience. The fact that the Adventurists still have a very personal approach with everyone taking part even though there were over 300 teams this summer is testament to how cool those guys are!
They are probably best known for organizing the Mongol Rally and the Rickshaw Run, among other charity rallies.
I participated in the Mongol Rally 2009 and for me was the experience of a lifetime.
This is their website if you are interested in learning more about them: https://www.theadventurists.com