Sorry to resurrect this necro-thread, but it was very helpful for me to decide to get the sliding tray. For me, the ability to have a flat area for sleeping was even more important than the functionality, but I wanted to add one minor thing that nobody has mentioned for future readers...

I always use my sliding cargo cover to hide my gear, especially when I'm transporting my guns. What I've found is that by using the cargo tray, I don't need to keep sliding the cargo cover back and forth to uncover the back when I load/unload. I can just slide out the tray and grab whatever I need and do the same thing when I load up. It also saves my back and knees since I don't need to move or load anything while I'm bending over - that can be a problem for an old guy like me. It was a 'must-have' on my list when finding the right 4Runner and will be in my next one too if it's still an option.