The X team has marked modular X stable for most architectures. The x86 and AMD64 architectures will update to X.Org 7.0, and other architectures will move to 7.1. Those two architectures are on 7.0 because binary drivers were broken in the 7.1 update due to a number of planned changes to the ABI (Application Binary Interface). Some remaining architectures are expected to go stable soon. To learn how to upgrade properly, read the migration guide before starting the upgrade. It discusses how the USE flags changed, why X.Org went modular and why emerge xorg-x11 won't be quite enough. Please file bugs at https://bugs.gentoo.org/ for any problems you encounter during the upgrade. Modular X is composed of a vast amount of changes, so there will probably be a few remaining issues during the first couple of days it's in stable.