There's one catch.
On the O2 UK deal, you can use the iPhone on 3G "direct" and browse to your heart's content. However, if you want to "tether" it to your laptop etc, you have to buy a tethering add-on at a very exorbitant price. I suspect the same restrictions apply elsewhere - I've not checked.
If you "jailbreak" your iPhone, that restriction goes away, as do several others. Most usefully, the car charger for your iPod (or any of those other chargers) will work with the iPhone. Non-jailbroken iPhones are very sniffy about which chargers they will allow to provide them with volts.