I've been on the fence about upgrading my 4S to the 5S. I know hardware wise the performance bump will be substantial. I am disappointed, however, in two major things.
One is the screen size, 4" is simply too small in today's age of 4.3-5" screens on other platforms. I recently played with a GS4 and was sad to go back to my 4S's pathetic 3.5" screen. The thought of having to live with a 4" screen for another two years is really disappointing me.
Two, and this really surprised me, is iOS7. I really dislike the OS compared to iOS6. I feel the overall usability has gone way done for me and I'm not a fan of the ugly grey color scheme in apps like Safari. I feel like I can live with iOS7 in the long run because functionally I can still get things done quickly, but I'm not enamored with the OS the way I was when I first switched to Apple.
Thus, I'm in an odd position right now. I don't feel like there is any phone on the market today that has what I want. Android as a platform has always been a let down for me. It's laggy, the UI is inconsistent across apps, and it crashed for too often for my liking. Windows Phone lacks the app library and some really basic features (e.g. notification center) it needs to compete....although to be fair a lot of this is supposed to be addressed in their 8.1 update.
Anyone else in my boat?
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If your last Android experience was laggy and crashy, then it's probably been awhile. You would be fine with a Galaxy S4/LG G2/Google Nexus 5 device, giving you a nice ~5" screen with a smooth, stable Android experience.
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As for iOS 7, other than the slower app animation transitions (opening and closing apps), and other than maybe hoping for more substantial changes, I'd bet that you will get used to it and even prefer it to iOS 6. Even as is iOS 7 is probably the biggest improvement to the iOS core in its history. I find it way better - nicer looking, and the stock apps are vastly improved (Safari with integrated search/address bar, app switching better, much better control center for quick toggles, cleaner overall appearance, etc.).
I'm so impressed at how a lot of my native apps have recently switched to the iOS 7 "design language" such as Kindle, Huffington Post and Evernote. I just find everything cleaner and easier to navigate with iOS 7.
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As to the size of the iPhone 5S, yep, it's a bummer. I jumped ship when I had a choice between a 3.5" iPhone 4 and a 4.3" Galaxy S2, and I haven't looked back. I'd consider going back to an iPhone if they released a 5" version, but otherwise I find it way too small for my tastes.