The lack of 3G support will be seen as a disappointment by many, and it's true that the browsing experience is hardly the fastest, even using EDGE. This is somewhat offset by the superb Wi-Fi implementation - I have the details of my home and office Wi-Fi networks in the iPhone, so as soon as it detects the network it connects and I've got high speed Internet access. Leaving the Wi-Fi antenna activated doesn't appear to have a massive impact on battery life either. In fact, I tend to leave both Wi-Fi and Bluetooth switched on all the time, and I've yet to run out of battery life. That said, I tend to have my phone docked with my computer much of the day, so it's constantly charging.
Just like in the US, the iPhone will be exclusive to one carrier in the UK, and that carrier is O2. Although I feel that this will stifle sales at first, obviously O2 is hoping that consumers will switch from other carriers as their contracts expire in order to get an iPhone. There's no denying that the iPhone is probably the best tool ever to illicit new subscribers to a network, but it also seems very unfair. I'm fairly certain that locking a specific phone to a specific network represents something of a monopoly, but I'm sure that O2 and Apple would argue that point. Either way, I'm all for freedom of choice, so, I've been using a US iPhone for a couple of months now with my T-Mobile SIM.
That's right, if you know where to look and what to do, you can unlock an iPhone and use it with any SIM you like, but Apple is trying its best to stop you doing this. The version 1.1.1 firmware re-locked all unlocked iPhones, but now that that has been cracked, the version 1.1.2 firmware is about to be launched, in an attempt to thwart anyone, like myself, who wants to use their own phone on their own choice of network. The truth is, that the iPhone works perfectly well with any SIM once it's unlocked, but your average consumer isn't likely to want to go through the process of unlocking an iPhone, so I dare say that the vast majority of initial UK iPhone users will just switch to O2.
O2 is throwing free Wi-Fi hotspot access into the bundle for UK iPhone customers, which is no small benefit. The Cloud network has around 7,500 hotspots, so you should be able to find a high speed Wi-Fi link without having to look too hard, and even if you have to use EDGE or GPRS, you won't have to worry about the cost, since all iPhone tariffs incorporate unlimited data. That said, McDonalds is offering free Wi-Fi at its restaurants now, so even without an O2 contract, you should be able to find free Wi-Fi without too much trouble - there are a lot of McDonalds branches out there after all.