I went to KD for their opening day this past weekend. I have been to plenty of legacy Six parks to experience their Flash Pass system, so I knew what to expect going in. The employees working the FL queues with the new phone-based scanners seemed to be well versed in how to scan people in, but the issues arose in those guests that entered the Fastlane queue before their countdown timers had expired. The employees were letting them simply stand in line until their QR code became available- forcing those who had their QR codes ready to go to wait several additional minutes- sometimes 10-15. Both TT and Pantherian were running one train (and kept shredding wheels... again), which compounded the issue.
The QR codes do scan quickly, and they do seem to have the FL queues adequately staffed. It was easy to make a reservation in the app, though like many have said I did not like having to have my phone on me. To lap Twisted Timbers, this required me to get my phone out of the locker every time I wanted to enter the fastlane queue- rather than just store my phone in the locker for several rides like you can with a wristband. For me, that is the biggest drawback.
I also experienced difficulty getting the all season FL on my pass to activate- after entering the park, the FL website still said I was not present in the park. I had to go to the premium experience center to get them to manually activate my FL reservations- which took nearly 30 minutes for them to figure out. I was glad Accesso was present to work through the bugs the first day.
I did receive a survey after my visit- where I also left this feedback.