I just updated my iPad 2 to iOS 8. An empty Cesium Viewer loads (and some Sandcastle examples as well as terrain work) but there are a lot of issues. The specs on the website show 1608 failures; many of them are non-WebGL related and show that some of the standards we rely on may not be implemented in the Safari (for example, the full screen API). It looks like the our Event object may be the cause of many of the failures. There are also plain old layout/rendering issues with CSS/HTML that we need to look at. Performance is pretty crappy, but this is on a 3.5 year old tablet. I would imagine performance would be much better on an iPad Air or mini. I’m not sure if newer iPads will fair any better in the unit tests however, since I’m sure Safari capabilities are the same on all supported devices.
Having official support for Cesium on iOS 8 looks promising, but we still have a lot of work to do before we can claim it. If anyone is interested in getting Cesium fully up an running on iOS, particularly if you have experience debugging iOS/Safari web apps, we would love the help. Otherwise I’m sure myself and others will look into this as time allows. Pull requests are always welcome.