Apparently, the servers are pretty hammered down during the first few hours of the release, according to one of the users in this article's comments. And it comes to a surprise, not everyone is impressed. One of my friends thought it's really buggy and slow. I would say expect that to happen during their first day. Not all programming skills are leet.
And there are other people who thinks the UI is awful looking. I must say though, the UI had been so consistent since the introduction of the original iPhone, just like Porsche 911s since the very first one. So while it is a milestone in the iOS family, yeah it is messed up, right from the comments from the obvious saying it's a total copy of Android.
Last of all, it's a shame that Apple chose to drop the iPod 4G in the update eligibility list. Least to say, that generation hasn't dated alot. All that nonsense means I'll be left out. So what do our users who has it at least think of it?
This is a bit stream of conscious, so pardon any grammatical, spelling errors, or repetition.
I downloaded it yesterday afternoon, and I have to say, I don't really mind it. Some of the default settings are extremely stupid (like showing all the music you 'can' download off of iCloud if you want, and the way that the home icons are laid out) but after a bit of tinkering, I seem to have it at a decent place. I agree with the lag. It's the same type of lag felt when first installing Windows 8. Too many animations!!! I'm sure it will run wonderfully on the stronger chips found in the 5 and 5S, but running it on my 4S, I am feeling a little last gen (can't imagine on the 4). I think with a little bit of optimization, it will be a very solid mobile OS, something to rival Kit-Kat. 7.1 can't come soon enough. I personally am not a fan of the UI, but I know that a lot of people are so I can't really comment there. Some features I can't understand why they took 7 iterations to introduce (like a built in flashlight and command center functions on the lock screen). Notification center is now a bit confusing, and the I HATE the way the music app performs and looks (despite iTunes radio being a decent idea, the execution was very poor). Apple made picking a song more steps than it already was, and added stupid pictures to the artist and larger album pictures that just take up more screen space. Cover flow is atrocious (but how many people actually use that?). The integration with Siri is much deeper and more robust, feels more mature than it did in iOS 6. Photos app is awful, works too much like iPhoto. Like the new camera, filters are a bit stupid and useless. Spotlight's new location is stupid. we have enough gestures already, and the access point for swiping down for spotlight is only 5-10 pixels below the access point for Notification Center. Safari is awful, but you already knew that ;). I like the new folders being able to hold more apps, don't like how they open in an entirely new window and are much more difficult to close. Like the new multitasking (the apps actually dynamically update while in the multitasking view). The new keyboard is by far my biggest bug, extremely laggy and a bit disgusting to look at (interestingly enough, if you don't update your 3rd party apps, they will still run the old keyboard while typing in them.).
Overall, I think that it is a major upgrade in terms of features from 6, despite the less appealing new UI. With a little optimization for older hardware, and a FIXED KEYBOARD, it will be a great OS, much more fluid and user friendly than iOS of the past.
I will probably keep editing as I think of more and use it more, but that is what I can think of off of the top of my head.
I have it on a 4 and it is not as bad as most think. Its fairly quick. The keyboard lags horribly though. Ive had it since beta 1 and it has been very stable since. I like it. Im fairly impressed. Since I have the 4 all I really got was Control Center and the redesign.
I've been using it since the Developer Preview came out. It runs very well on my iPhone 5, but rather slowly on my work iPad 3rd Gen.
I'm not really that fussed about the UI changes since I've been running custom themes and icons for years. It does look a lot nicer, but it still feels a bit busy. The core functionality hasn't really changed outside of the Control Centre. I'll still be looking to jailbreak tweaks; assuming it doesn't take ages to jailbreak for the iPhone 5.
Some the apps have started updating to match iOS7's look and feel. Some are much more functional (like facebook and Reeder 2) but some are just a coat of new paint (Twitter and Chrome)
Overall, I'd give it a passing grade, but it wasn't the leap I was hoping for in the end.