I tried using it the other day, while I had threads that I hadn't viewed yet.
I clicked "Mark All Read" and it redirected to a blank page.
When I clicked Back in my browser interface, and then Refreshed the page, the threads that had been marked as to be read were now marked as read.
I thought the only cause for concern was that it had redirected to a blank page. But perhaps it does need further testing, maybe I am wrong about something.
Personally, I get taken to a blank page, but all of the subforums' threads are not in fact marked as read. I still have the QMC2 forum showing as having unread posts, for example.
I also hate the fact that "Reply" and "Quote" take me to that crappy "Quick Reply" box at the bottom of threads now. I want to be taken to the separate reply page like I was before.