Was there supposed to be a splash screen after updating? I didn't get anything, just took me directly to the home screen. I never saw the Message in iCloud splash screen either despite enabling it.
Enabling MiiC is a bit wonky. Currently after uploading messages from both my iPhone and iPad, the iCloud storage listed for Messages on both my iPhone and iPad is showing as 270 MB. My iPad is showing 92 MB of conversations, while my iPhone is showing no conversations. My iPhone is showing Messages local storage as 105 MB while my iPad is showing 14 MB.
Deleting threads syncs, but deleting individual messages is hit or miss (sent messages delete, but received do not). Things haven't synced up, but I guess that just takes time.
Something I find odd is that Messages in iCloud doesn't appear to behave like Photos in iCloud. Messages don't show up under iCloud Backups, but there's no messages saying that's the case like for Photos in iCloud. Also Messages take up a lot more storage in iCloud than they did on the individual devices.