For me it works perfect and have done so since 11.3 beta.
I have an iPhone X, iPad Pro 12.9 and a MacBook Pro. Imessages in iCloud works great between all these three units.
When I first turned the feature on, I had a mess with different messages on all three units. After 24 hours all units where synced perfect and had access to the same complete messages. The best feature of them all is that one message deleted on one unit is deleted on the other two units - I love it and wouldn’t wanna be without it now.
However, note what I wrote in another reply. For the feature to work you need the same beta version on all units. If not, it won’t work. Let me explain: I had IOS beta 1 on my iPad and macOS beta 1 on my Mac. When updating to IOS beta 2 on my iPhone, iMessages in iCloud didn’t work anymore (only between units with beta 1). When all units where updated to beta 2, everything was normal again. The later could be important to know since Apple releases beta versions for different units at different times. So when you see people in this thread saying iMessages in iCloud don’t work, it could be related to that.