Works fine on my 5s. Lagging was pretty much gone by 8.0.2, and 8.1 seems to have solved the Bluetooth issues. Occasional crashes from apps that had always given me problems. Otherwise, none of the random reboots that I had with iOS 7.0 to 7.0.3. On my phone, the performance and stability are at least on par with 7.1.2.
I generally prefer to do the updates using iTunes, and I also restore my phone to factory settings prior to any major interval updates (8.1 would be one of them). If everything checks out, then I restore my apps, settings, and data from a local backup using iTunes.