I have the 7yr/100k mile Platinum warranty on mine. When I bought it from the dealer, they offered a money-back guarantee on it if I don't use it. The finanace manager said, it was a new program they started offering to make the extended warranty decision easier.
For the 4th gen mostly... If I were to take a guess, I would say maybe 20% of the members here have an extended warranty. For those that do, it's been a good ROI (return on investment) for them. Most of the time, 4Runners don't have many issues, but when they do it, they're easily four-figure repairs. One of the more expensive and probably more common repair out of warranty is the moonroof. A few (by no way an alarming rate) had X-REAS replaced.
The bright side is even with a new generation 4Runner, you have some time to decide if you need the warranty in the next couple years and see if there's a common theme of problems that can develop. (What I mean is with the 4th gen, common possible problems were the moonroof rattling or it stops closing, seat popping, driveshaft thump (still no true fix for 4Runners), seat popping/movement.) Toyota's extended warranty through TFS (Toyota Financial Services) can be purchased at any time you're under the 3yr/36k mile bumper to bumper warranty.