Like Drusstw, my 8X with 8.1DP battery life is not great otherwise 8.1 has been pretty smooth. And, starting yesterday, my phone has been shutting down expectantly...this might be the SIM tray issue again.
The battery life on my 8X (Three UK) was appalling and it was get insanely hot initially so I did a factory reset and now it runs normally and I get pretty much what I did with 8.0.