I'm waiting for my iPhone 6 to arrive next month, but in the meantime, I downloaded iOS 8.0.1 a couple of weeks ago to my iPhone 4s. Big mistake! I knew the operating system was much larger than iOS 7, but I figured that my 64gb phone could handle it. It couldn't.
The phone suddenly worked much more slowly and with a lot of aggravation. Switching between apps (by double-clicking the home button) didn't work correctly, and some of the apps would get stuck so badly that I'd have to re-boot the whole thing. Plus, Siri seemed to have a bad attitude, like I was making her work overtime.
The biggest problem was that the bluetooth I used in my car wouldn't connect, or if it did, it connected for audio but not for phone calls. Many times, I'd get in the car and the handshake between the vehicle's audio system and the phone's bluetooth would cause the phone to ring once and then pause, locking up the whole system for both audio and calls. I'd have to disconnect bluetooth, turn it back on, then choose the car's connection again and again. A huge pain.
But I'm happy to report that Apple has fixed those problems with the new iOS 8.1, which became available this week. After downloading it, I'm not having any significant problems with the 4s.
I mention all of this because many of you may also have an iPhone 4s, and I'd suggest that you do the same thing I told my wife -- do not download any version of the new operating system, particularly if you own the 16gb version. Also, because most of the apps have been updated by their providers to work with the new operating system, they may be incompatible with your phone running on the old OS. To avoid those problems, go into Setting > General > Background App Refresh and turn it off.
I wish someone had told me this two weeks ago so I could have avoided these problems.