I am a US student with a fully funded offer to do my PhD in the UK and although was very happy to get the offer, the more I talk to people the more my enthusiasm is fading. Here are my concerns:

After I am done with my PhD, I want to get a teaching job in the US and I have had many people tell me that unless is Oxford or Cmbrige is very difficult - almost impossible, especially since the uni I got the offer from ranks in the mid 30's. So my main concerns: post-phd employment and university ranking - how much will these things genuenly matter?

thanks!!!

In my experience, university rankings don't really play a major role in post graduation employment.
Oxford and Cambridge are in a class of their own but many other British universities are also well regarded all around the world.
Employers tend to view international education very favourably, maybe because of the diversity between teaching experiences between different institutions in a variety of countries.
Studying in the UK ought to make you more employable not less.

Did you select a course that really appeals to you?
If so, stop worrying and enjoy the opportunity.
If you're feeling out of depth at the prospect of coming to a new area look the place up online so that you can get a taste of what living there will be like.

Congratulations on getting funded. All the best with your PhD.

Mmm what sort of college would you want to teach at? I have heard SLACs are not keen on hiring non-US PhDs because we're too research-focussed in Europe, and community colleges are even worse because they don't want the expense of flying someone in. If a more research-based career then I think it would come down to the PhD project itself - can you give a good reason why you did it in the UK and why this university? E.g. access to archives, supervisor, area where UK has research advantage. It goes without saying that you'd have to have publications, teaching experience etc like any US candidate but you need that for UK universities anyway. Final thing - do you already have an masters in the area you'd want to teach in? I think some states demand so many completed taught postgrad courses for you to be able to teach at a college.