EE are the only provider in the UK offering LTE/4G for Apple Watch 3. Since I was already an EE business subscriber this was alright for me (jack!). However getting it working as described has not been the easiest. I won't detail all those issues here suffice to say that the online press have already picked up on them and many people appear to have suffered.
That said business and level 2 support from EE have been excellent, calling me back when they said they would, and generally working through the issues I have had until it is now all working correctly. If you haven't spoken to them and are having issues, I'd recommend starting a dialog - they are certainly keen to help, and L2 support is technically excellent.
However the larger impression I get is that EE have been taken by surprise by the nuances of setup for many, and their technical support team are working on each case one by one; learning as they do.
Is this a result of poor preparation from EE, or from the way Apple give EE access to the technology ahead of the launch? Only Apple and EE know that, and I am sure they have both learnt from the potential repetitional damage that could come from this. These things gradually wear down trust in a company; nobody actually want's to deal with support no matter how good they are. Although I expect it will be forgotten or not even noticed by the majority of users a few months down the line I still feel let down by the experience.
However, after a couple of weeks of frustration, I am now able to go out on a run, bike or swim without having to carry my phone with me, yet still be able to contact my wife (e.g. "help I have broken a spoke") if need be - that's nice (for me if not for her 😉)