Regarding mobile: the latest Android Browser, and the mainline Chrome for Android and beta Chrome for Android both have CC20. agl specifically called out Android as a use case - ARMv7+NEON performance of ChaCha20 is substantially faster than, say, AES.
Yes, you'll probably be waiting until the RFC is out for wider support. I reckon it makes a leading candidate for "AES backup/standby", however.
(And don't get me wrong: I LOVE the work that's being done, and can't wait to be through with older cipher families!)
If you're streaming to mobile, ChaCha20 is out.
IE, Firefox, and Safari on the desktop: out. (so far)
Compatibility with IIS or any non-bleeding-edge middleware servers: out.