I was actually really interested in AppleCare One. We have small kids, it'd save me a few bucks, and it'd make things easier as we cycle devices. Sadly, reality hasn't lived up to the promise. Between the bugs I can't get past and obvious use cases they haven't yet supported, I have no use for this.
Of the three devices I currently have enrolled in AppleCare+ and two more that I considered enrolling in this, all of which are eligible for AppleCare One, I'm only actually able to enroll one of them in AppleCare One.
What a mess.
Details below...
1) With AppleCare+, you can pay for a subscription even if device isn't linked to your Apple account. E.g. My mother's Apple Watch Series 10 is linked to her account, but the AppleCare+ subscription is linked to my account: it shows up in my list of subscriptions in my Settings app because I paid for it using my account. With AppleCare One, that isn't a supported use case, apparently: the Subscriptions screens in Settings provides no mechanism for "upgrading" to AppleCare One, her device isn't available to select during AppleCare One onboarding on either web or via Settings, and when I tried to manually look up her device via its serial number the website didn't provide me with an option to upgrade (it instead just told me that her device isn't eligible for coverage, presumably because it's already covered by AppleCare+).
2) Another device I'm paying for is linked to my wife's account. She's a member of our Apple Family and uses the same credit card for her subscription that I do for mine. Despite that, there's no means by which to migrate her iPhone into a plan linked to my account. She'd need to have her own AppleCare One plan separate from mine, or else we'd have to unlink her device from her account, link it to mine, sign her up...no, just no. I'm not jumping through those hoops.
3) But hey, maybe I could toss my aging iPhone 13 mini or our M1 MBA onto the plan instead? If the price is right, why not? The web interface doesn't present me with the "Upgrade" option like I see for my iPad Pro that currently has AppleCare+, presumably because they aren't on AppleCare+ right now, but Apple's documentation says that both should be eligible for AppleCare One after a remote device check, and they're available to select during AppleCare One onboarding via the Settings app, so all good, right? Nope. The Settings screen immediately throws a "Something Went Wrong" error and drops me out of the onboarding flow altogether. I don't know if it's a bug with handling any devices that aren't already on AppleCare+, or if it's a bug with devices that require a device check. Either way, it's broken.