I have the second gen iphone for the last 8 mths, so basically its still the 3G but doesnt have the issues that the first ones had. In terms of battery life ive heard both sides, mine will last for a full 4 days before i need to charge it. If i use the apps non stop, which im guessing you wont be, then it will still last for 2 days minimum. My friends needs charging every day however but when he first bought it he didnt charge it properly so i think thats what its down too.
The sat nav is awesome but dont go for the TomTom, theres another one called Co-Pilot which is much better, and as long as you dont mind it being on a smaller screen its pretty good.
If you want a good deal wait for Orange as it will bring both companies prices down finally. I currently have to pay £45 a month for something my friend gets at £30.