This factory image will be quite helpful in a while, because it will let you reset back to stock if you jump down the rooting and custom ROM rabbit hole. Unfortunately, custom ROMs for the Nexus 9 are still a while away, because the source code alone isn't enough to make a working ROM. We still have to wait on the driver binaries, which can sometimes take a while.
We haven't seen any other factory images push out, which isn't much of a surprise given that Google hasn't started the rollout of Android 5.0 to any other devices. Usually the factory image will come around the same time as the OTA rollout. Google has claimed Android 5.0 would be coming to Nexus devices soon, but we won't know when until it happens.