Well if you haven’t already got an iPhone I don’t think there’s any question about it – I love my iPhone – go for it!
If it’s the lte (long term evolution)/4g you are after then currently it will only work for speeds if you buy the phone on T-mobile and go to Germany! Although here in the UK we are awaiting the joint venture of T-mobile and Orange EE (formerly known as Everything Everywhere) to flick the switch on a 1800mhz 4G network by reusing some of their old 2g signal.
To you and me what that means is if you buy the UK/Europe launch model then the 4G will work on Orange & T-mobile (after their switch). Three will have a small chunk of this spectrum too, but O2 and Vodafone are planning on using the old analogue TV signal (2600mHz) which is due to be auctioned off by Ofcom next year, but if you buy the launch model on these networks it won’t be compatible with their 4G unless O2/Voda manage to get 1800mhz too
Other things to consider are of course you will need to replace all docks, connectors, etc; there’s a new nano sim – so if you want to stay as pay as you go this may be more complicated/sim need replacing; there are new maps with traffic updates – road works etc but the maps are rumoured to be not as good as the previously used google maps – which have been criticised for not providing directions for public transportation; the screen isn’t a true 16:9 widescreen because the resolution isn’t correct.
The camera is now back illuminated so you should get better night time/low level lighting photos.
The price of the 4s started really high but went down quiet quick due to competition – so it may be worth waiting to see if the iPhone 5 comes down in price.
Some of the features of the ioS6 are available on previous iPhones – but not all.