Mine works great except when its lying flat or I'm not looking with my eyes (i turned the feature off to). While sometimes annoying, it is what it is and will only get better.
To be honest, one complaint i have is the change from a 4 to 6 digit passcode. A lot of times i'll be at my desk and want to unnlock my iphone, i know face ID wont work because its lying flat so i have to enter in a passcode. Given how many times a day i do this, a 6 digit code is just annoying. I find it hard to believe that someone who would/could steal your 4 digit code would have a much harder time with a 6 digit code... I wish we could change it/have the option.