Error reporting and documentation are incomplete, and there’s bugs and other issues not yet resolved, but the AE and OSA support almost all works so folks already trying out JXA on Yosemite will find it useful for comparison. Folks on Mavericks can also try it if they wish as it’s 10.9-compatible too. I’ll try to finish it later if I’ve time, but as Apple have deprecated the whole OSA foundation it’ll never be suitable for production use; therefore it’s JXA or bust.
At this late stage I think the only solution is for users to demand Apple reschedule JXA’s initial release for 10.11, giving them time to [re]do it right. Bear in mind that once 10.10 ships, JXA’s design defects will be effectively welded in for good. So it’s up to all of you to decide if you’re willing to accept something that’s crippled, broken, and thoroughly inferior to AppleScript, and to make yourselves heard by those in charge if not.
[cross-posted from the AppleScript Users mailing list]