No issues at all on mine. I'm running my 5S back-to-back with my 6 and yes of course it is slower than the 6 but only by the barest margin. Handoff/Continuity seems to be working fine between the two and my iPad now too.
After installing it, I had 39 app crashes, most of which were behind-the-scenes items. But Safari would crash on me almost any time I tried to open a link from the Facebook web site, the Music app crashed a few times when what it apparently forgot what the last thing I was listening to was when I restarted the app after closing it down.
And iOS8 appears to be about a gigabyte larger - I have a 16GB iPhone 5, and it went from a smidge more than 13GB available to the user to about 12.
Updated two days ago, everything is running smooth. Battery is terrible. I pulled my phone off the charger @ 9:30, its now 12:30 and I have 9 percent. I already did the battery swap through Apple earlier this year. Gonna have to do some digging around, but I haven't changed anything since iOS 7.
I cleaned up my ip5 before updating and then took the plunge. Needless to say i have been less than ecstatic about the laginess and phone resetting every few hours. Only a few apps crash ie Facebook Safari and flipboard. But at least I get my ip6 on Wednesday.
Mine is running okay, but battery is horrible. My battery was already pretty bad with iOS 7 and the update just made it worse. I don't feel like shelling out money for a new battery since I'm close to my upgrade anyway. I get maybe 4 hours of usage if I'm lucky.
Switching between keyboards (I have three, English, Emoji, and Korean) is kind of laggy and doesn't always register so I have to hit the globe key several times. Third party keyboards are also especially laggy for me, but that's probably an app issue and not the phone or iOS.
Update is running super smooth. I noticed that my battery life was super unstable at first, but I restarted it a couple of time and ran it as normal for a couple of days and it seemed to work everything out. I would say that my battery life is a little worse than IOS 7.