Yes, we know iOS 8 will shine on iPhone 6 and iPhone 6 Plus, but so far we have a Gold Master of the next-generation iOS that runs on the iPhone 5s and that doesn't work well.
iOS 8 is not a rushed operating system. Apple had one year to improve on it. However, it seems that not enough testing has gone into it, so there are still hiccups in the way the GM version works on an iPhone 5s.
The next-generation iPhone and iPad version is scheduled to be released on September 17. Apple has released the developer version of the Gold Master 8 days in advance so if you really want to test it, you can find it online. You don't even need to be a developer to install it. Just connect your iPhone to iTunes and press option (alt) + Restore.
Admittedly, iOS 8 is the iOS 7 we always wanted to have and the bugs are still to be corrected until the final version drops a week later, but Apple is known for their attention to detail and having an iOS that has issues on their current flagship phone is not a good thing.
We have tested iOS 8 GM for a couple of days now, and the initial impression was positive. It runs as smoothly as iOS 7 and it brings some improved functionality.
The Good
The Mail app experience has been greatly improved with notifications, flags, and swipe gestures that reveal new actions. Also, the multitasking interface now has your most recent people that you interacted with, so you can just go from there when you need to send a quick message or call someone.
The Keyboard's Dictation and Siri features are much more responsive. Also, with iOS 8 you are going to have the option of adding 3rd-party keyboards. TextExpander and Swift Key are some of the most expected ones.
The Bad
iBooks is now a default app, as well as Podcasts, Health and Tips. And since they come with the iOS, you cannot delete them.
Siri still doesn't have Shazam integration, as promised, so you cannot recognize a song using iOS. Apple may introduce the feature in an iOS 8.0.1 or maybe they have left it for the final version.
The Truth
When you first install iOS 8 GM on an iPhone 5s, you are going to notice that some apps have a certain lag when you first open them. Also, if you choose to restore from an iCloud backup, you are going to run into some strange issues. Even after 48 hours, the testing phone we've used did not recover everything, iOS 8 thinks, although all the settings, apps and content was restored.
Receiving more than one phone call at a time may confuse the phone and the screen can get stuck in between lines.
The conclusion is simple: we are really looking forward to having a bug-free iOS 8, and hopefully that will not come two months later in the form of iOS 8.0.1.