Installed it on my 4S today. Initial impressions:
The Bad:
1. Hate the whiteness. Reminds me of Office 2013. Terrible. Too bright. I need a dark gray option for stuff like Safari & Photos. The white keyboard in texting is terrible to look at, I liked the old gray one so much more. Hate the Mail app visual design too, it's now worse to look at than the Gmail app. 100% white with an animation style I don't like, soft-snap or something.
2. I don't like the slightly slower, Microsoft-style animation. Reminds me of Windows 8, although at least Windows 8 is snappy. Everything just feels a bit laggy, but not in a hardware laggy kind of way, just in a "we designed it like this" sort of way.
3. Again, need more options with color schemes. No solid-color background choices by default. The lock screen is white - I put up a family picture and I can't read the time. Maybe they could offer a black-text option, or a white-text/black-border option. A lot of the graphics & text simply lack good contrast and they are hard to read.
4. Don't like the flat icons or rainbow color scheme. I don't know what they were smoking. I mean, I guess it could look good in theory, but it only feels like it's 90% there. With the default blue clouds wallpaper, the Mail icon blends right. The Settings gear icon looks terrible. The App Store icon's circle is a little too big for the icon square. Little stuff like that just doesn't look quite right.
5. It added a Facetime icon to my homepage. It shoved the last icon on my homepage off to the 2nd page. It pushed everything on my second page over to the third page. So now I have an almost blank second page, with all of my 2nd-page go-to tools on the third page. To put this in perspective, I have 11 pages of apps organized exactly how I want them. I now have to go through and move nearly 50 application icons by hand.
6. Still no "close all" button on the kill-apps screen.
7. Too much stuff feels like Droid in this. For starters, the front of the time on the lock screen. And then the swipe-anyway-and-something-happens. I mean, I'm used to it now (slide up for control center, swipe down for search, etc.), but little bits & pieces have that cluttered/poor graphic design feeling of Android. Like the bars next to the Verizon 3G at the top are now little circles, a design feature that matches absolutely nothing else in the interface. Easier to use? Sure. Weird to look at, especially compared how doesn't fit in with the rest of the design? Yup. I've been using an iPhone since 2007. I'm used to things a certain way, so if you're going to change things, do a 100% job, not a nearly-100% job, you know? Yes, it's finicky to point it out, but Apple is usually great about attention-to-detail.
8. Still no Swype or Swiftkey or other multi-touch solution to make typing easier or faster.
The Meh:
1. The new app-kill system is more user-friendly, but they also combined it with app switching, so that's interesting. I actually find this a little harder to use when switching to another running app, because I knew which icon I wanted to open before, and now I have to spend more time looking at the preview trying to figure out which one it was. Not a big deal since the icon is still underneath, but it's no longer in a black strip anymore. Not a biggie, just different.
2. Siri is kinda cooler. Still more of a novelty than anything for me tho.
3. New Camera tools, meh. The filters are not good quality and are fairly boring. Sticking with Camera+ myself. I also keep accidentally swapping formats in the Camera app when I don't want to, like to square.
4. No AirDrop on the 4S. To be expected since they want to deprecate features and make you want to upgrade, but still, lame.
5. Maps is really coming along nicely - surprisingly. I still use my dedicated Garmin in the car the majority of the time, but it's another nice option along with the Garmin app & the Google Maps app.
The Good:
1. Pretty much all of the bad stuff is made up for the new slide-from-bottom control center. I REALLY appreciate this, especially since I turn my Bluetooth on/off every time I hop in the car to save battery. The flashlight feature is also most excellent. Again, not crazy about the fuzzy rainbow garbage going on behind the interface, but whatever, it's super functional and is a BIG help in how I use my iPhone.
2. Battery life seems to have improved for me, doesn't die nearly as fast now. That's only first-day impressions, so who knows, placebo effect maybe. But it's nearly midnight and I'm only at 40% after using it intermittently all day.
3. The slide-down-from-top to open Search iPhone from any icon window is handy. I keep accidentally opening it and will have to un-learn this 6-year habit.
4. Slide-to-unlock from anywhere is great. Much more natural.
5. I like that the notification center now has tabs and shows today's calendar, but you can also click on All (or Missed). Would still prefer an Intelliscreen type of setup, but this is a start.
Bottom line: hate the white, hate the lack of contrast; love the quick-access control center, love the slide-from-anywhere-to-unlock improvement. It's only been a day and I'm mostly used to it. There are some things that offend the designer in me, but oh well, it's a phone.