The iPhone Xs Smart Battery case worked perfectly on my iPhone X. There is nothing like perfect integration with Apple's ecosystem that tells in in native control panels what the charge level of both the iPhone and the Smart Battery case is at. And the battery life gets me through the entire day, something that I've almost never achieved with every single iPhone generation (with the way I use the phone).
It isn't without its flaws though.
The white version will likely end up yellowing, and looking dirty with age and use.
The black version looks greasy if you have greasy and sweaty hands.
Both are not weather resistant rated in any way.
Lightning port on it is not fully compatible with all accessories, but at least certain important ones to me like the lightning headphones, charging work with no issue.
Finally, there hasn't been a great way for me to attach a metal plate on the back of one for my in-car magnetic mounting system yet.
Overall, if you can overlook a few shortcomings, I'd say the case is worth buying.