I'm doing a taught Master's at a Russell's group Uni after an undergrad at the University of California. I'd say that many of the preceding posts seem to be rationalizations and then some are actual experience. While I have yet to graduate (let alone take my MA for a test drive out in the job market) there is one thing I know- you have to first of all apply and then you have to sell yourself. It's not so much the degree, it's who you know and how you present yourself and ultimately taking the degree and the knowledge gained and applying it to whatever job you are doing.
Students graduating from my department are like students from any school stateside- the ones who are good at identifying goals and working toward them are not sorely disappointed that they took a degree. The ones that looked at the degree as a bonafide credential or a ticket to the next phase... well it doesn't seem to work that way.
Bit of truth- social anthropology (that's what they call it over here) and the degree are constructed to be a step in the career pathway to being a professor. The demand for teaching staff... well, um "would you like fries with that" is something to think about- the structural faults of the education system both in the US and the UK are now at point where blaming the institution, the individual or the degree itself is just plain stupid. All the notions of education= something tangible... well, that probably hasn't been the case for years. Conversely, degrees still help get jobs- finding the balance and leaving behind the myth really helps.
We went and became educated- we can write, work with ideas with a great deal of sophitication and therefore are employable.
The sooner you identify exactly what your goal is and exactly how the degree benefits you along with the chances of successfully completing the degree without a huge debt the better the experience will be ( hint: if you're already in debt- taking on additional tens of 1000's is worse than smoking meth- it just is, don't argue).
My reasons were very individual and almost unique- my degree at a well regarded university in the UK demonstrates my ability to adapt and excel- something that people in my age bracket are said to be lacking. Moreover, ( and this is totally my opinion) the UK education is settled on the basics. Because really it's a lifestyle and a culture that you adopt, not a manditory set of meetings and exercizes that are rigorously graded- something that I could not sometimes convince other members of my cohort. Education isn't always neatly graded and within a rubric- it's what you do with it.
If you want to do well- show up at all the seminars, present at and attend conferences. Work on publishing- avail yourself of the opportunities, go to after seminar drinks at the pub. There is a whole world of learning that doesn't follow US guidelines. And these folks in the UK are just as accomplished.
PS- BBC article pegs the US as # 14 in higher education ratings- the UK is #6. http://www.bbc.co.uk/news/business-27314075. That factoid is important when judging what your degree might be "worth".
I personally would not bother with a program that is going to disallow my degree and make take another Master's - if I do a PhD I will do it in the UK at a top tier school because really Oxbridge, LSE and SOAS are not lesser schools - anywhere in the world. The British work hard to keep their standing and pedigree- something that will not change anytime soon- if you want to go to the UK, quit reading this board and find out for yourself. It might really work well for you.