I've just made the move from an iPhone 4, and I have to say that I'm not enjoying it much.
The phone is amazingly fast, and has lovely industrial design and construction, but the software is just dreadful (at least for the things I want).
Things I've found don't work properly are:
1) Camera is dreadful in low light, and crashes regularly when I'm using burst mode.
2) Bluetooth is fundamentally broken in several areas.
- I use the phone a lot in the car in conjunction with a Pioneer AVH-X8500BT. This is a great multimedia head unit, and has Parrot bluetooth and AppRadio features. With the iPhone, you simply pair the phone, connect the dock cable, and it just works. With the Z1, the contact book is completely messed up, as any contact that just has a company name only shows the telephone number. It's impossible to work out who you are dialling. Bluetooth audio playback is also broken with pops and breakups. You also get no track information showing on the display, which the iPhone and other Android phones I've used do.
3) AppRadio features don't work. The unit connects, and you see an image on the head unit, but can't control anything. I've spoken to Pioneer, and they claim the fault lies with Sony, but Sony aren't interested. I'm told that ARLiberator might fix this, but I'm not sure I want to drop £25 and then find it's still broken.
4) Poor battery life.
I really like Android for it's extra flexibility over iOS, but all I want is a stable software platform, and Sony just aren't delivering this on the Z1 at the moment. I'm acutally wishing I'd gone for another iPhone, as I know all of the above features just work.
I'm also really irritated that Sony haven't pushed out any updates to unbranded, unlocked handsets. I can't begin to fathom the reason when other carriers (O2, for example) are pushing out these updates.
If I was you, then I'd stick with your iPhone 5, and wait to see if Apple really change things with the iPhone 6 next year.