I have tried using Renderscrip in a large scale application (+1 million daily users) with a min API = 7. It is in fact available through the support library from API 8.
However :
-The Gaussian Blur Intrinsic is buggy on some terminals ! The implementation of Gaussian Blur in the Nexus 10 makes a mistake in a division (1000 instead of 1024 or the reverse), so we have very visible artefacts on this terminal. It might have been solved with the Lollipop ROM though.
-Using Renderscript on old terminal has proved to be a real pain in the ass. Since we have a large userbase, we also encounter some fairly exotic terminals. If I remember well, some Wikio phones (low cost chinese phones, but validated by Google) crashes as soon as you try to invoke Renderscript in a native thread (so you don't have any way to recover from this exception).
In the end, we removed Renderscript and replaced it by a custom blur implementation. In our case RS was not used for real time blur rendering, just blurring of a downloaded images, so the computation time is not vital.
I might have tried to reintroduce Renderscript later on, at least for the latest API levels but now that blurring does not fit anymore in Android's design language, we will just remove the blurred views from our app.