I used to enter them when they just started around 6-7 years ago. They started out on the Sony Talk Forum (STF) on dpreview as a weekly challenge, but soon became too big to be a weekly event and changed to be a monthly one instead. They then spilled over to the Nikon and Cannon forums, and I can now see that they are a big part of the website.
The winner used to win the honour of calling the next challenge and hosting it (quite a lot of work, somewhat glad that I didn't win), don't know what the winners win now. I even managed to get a shared 4th place, pretty darn hard since there are plenty of very good photographers on dpreview. I learned a lot there and when my Sony F707 died I didn't go back, need to get back into it as I really enjoyed it.
Here is my 4th place shot
Things in Motion challenge #8, they are now at #186 on STF.