It's ready to go. We sent it to Apple last Thursday. This is the most busy time of year though for them especially this week. It could go live in 5 minutes or 5 weeks. After 7 years and 39 days on the App Store, I've learned never to try to predict the App Store timing.
Siri Custom Intents was both better and worse than I had hoped last June. The best way to describe it is to say I simply use it differently than Alexa. I would say I use it as often and perhaps for more important scenarios than Alexa. Alexa is more flexible with what you can say, but with Siri CI you say exactly what you want to say so you don't need to guess what syntax Alexa might be wanting.
How I might wake up with Alexa:
"Alexa, turn on Great Room Waffle Wakeup"
With Siri CI:
"Waffle Time" -- I tend to use "hold to talk" on an iPad home button for Siri, but that would work just as well with "hey Siri...".
Alexa has a semi-predefined syntax that we provided broad support for so you can say something like "Alexa, tell Simple Control to tune HBO in the Living Room." With Siri, you'd need to put that into an Activity, but you'd then be able to say simply "HBO" or anything you want. Alexa does not have a "say anything you want" mode.
There are a lot of little differences. In the end, I like it. I prefer it for some things. I might prefer it for more if I didn't have years of brain muscle memory for all the Alexa commands. If you don't use Alexa now for Voice Control with SC, I think people will love it. Some people will use both. People in the many countries not supported by Alexa will be overjoyed.
One interesting difference is that Siri CI does not require a Hub, so if you just use one controller, more people will be able to use voice control now. Every iOS device is independent, so if you say "waffle" into a particular device, that device needs to have been setup for exactly that. On one hand, that's better because "watch TiVo" can be setup on each room's iPad so that there is no need to specify the room in the command as with Alexa (eg. "Watch TiVo in the Living Room"). On the other hand, it means you need to setup a fair number of commands on each device and think about whether an iOS device moves between rooms in which case you may need to vary commands or add the room name back.
The fundamental piece Apple hasn't implemented yet is understanding what the user is trying to say from voice. They will match your pre-programmed voice recording with the Siri CI you created and it executes instantly. What they do not do is voice syntax parsing like "Alexa, tell Simple Control to set the volume to -32.5 in the Master Bedroom." For Alexa, that command first determines the skill, Simple Control, then picks the command to 'set volume', then provides a variable value '-32.5', and also provides a variable 'Master Bedroom'. Alexa then accepts multiple renditions of that command. While deep support for all of that exists at the non-voice level (ie. Shortcuts app) in Siri CI, no syntax parsing exists at the voice level. That is a sizable gap between the two systems right now. We provide specific intents to mute, unmute, etc for the whole room on any Activity that should cover most use cases.
Let us know what you think (after it's released) and what other intents you'd like to see. We focused on the voice experience in this release. No doubt there are more intents we can add with variable parameters at the non-voice level that will be useful only for Shortcuts.