WinterBoard 0.9.3912 (just released) runs on iOS 7 and seems compatible with ARM64; theme artists are encouraged to report any crashes/bugs.
If you are waiting to jailbreak your iOS 8.0-8.1 device until things are "stable enough": we now seem to be ready! ;P en.pangu.io
The TaiG 8.1.3-8.x Untether, version 2.1.2, supports Substrate! This package is available as an update in Cydia (from our Cydia repository).
Cydia vendors can now mark their products as iOS 8 compatible. (The jailbreak platform itself is now stable enough to make this reasonable.)