One thing that no one seems to have mentioned here is that according to the Apple website the watch is 4G/LTE ONLY in the UK. There is no fall back to 3G/UTMS on EE unlike almost every other network it is being launched on. This makes the thing basically pointless if you live outside a city. I live in a small town and while we have 4G as soon as you get outside of the cell tower covering town centre your back to fast 3G. I would really suggest checking your network coverage if your going to use this as a smart fitness tracker and leave your phone at home.
I would have liked to use it when I am out cycling and stuff but there is no 4G when I'm out and about so there is no point as it would need to be tethered to get the online features. It is also very clear the cellular is going to destroy the battery.