There are downsides to 20mm, though. The big axle means big hubs and plenty of metal on the forks, which all adds weight. Working on the basis that much of the stiffness (and all of the security) benefit of through-axles is due to the axle passing through a closed "dropout" rather than the diameter of the axle, Shimano and Fox have introduced a new, lighter, standard aimed at all-mountain and XC use.
In use the 15mm system is a little like SRAM's Maxle setup, with an axle that threads in to one side of the hub and has a QR-style lever to lock it all in place. Unlike Maxle, though, the threads in the hub are actually a splined insert, so you can adjust the final lever orientation by moving the insert round so that the thread starts in a different place. The lever cam exerts a clamping force on the outside face of the "dropout" to lock everything in place, rather than Maxle's expander that wedges itself inside the dropout.