Qt5.9 is now available in testing along with Plasma 5.10.1.
Tha mayor thing to notice is that is a LTS release (Long term supported). The webengine backend was updated to Chromium 56, more speed, ton of bug fixed. I expect to be a good release, very stable (even more starting from the next bugfix release)
Plasma 5.10.2 is also ready in testing, build against the new Qt5.9.