This is a really interesting topic - I kept planning to do an MA in creative writing but various reasons have made me eventually decide on doing an english MA. I've met so many people on my undergraduate degree (in english) who bang on about how much they love creative writing, and then you read their stuff, and hear them speak about their plans for the future, and realise how little they know about the actual 'business' of creative writing. I think Angelil is correct about the majority of these students having delusions of grandeur. I think, if you want to write poetry or fiction, the very best preparation you can do is to do an English Literature degree, not a creative writing degree. I know lots of wanna-be-poets, but if I ask them what contemporary poets they read, they won't have a clue (and also won't care, being obsessed with their own poetry and ignorant to their contemporaries). A passion and interest in what other people are writing is something that a Creative Writing degree won't teach you, but what an English degree might. Also, although there are a few well-known fiction writers and poets who have MA's in Creative Writing, what writer do you know of who did an ungergraduate degree in Creative Writing? I can't think of any.
I can understand why some would choose to do an MA in creative writing. I think there are two types of creative writing students, as the_alba described - the majority are living on their own planet who haven't got a clue about anything else other than what dribbles out of their minds onto the page. But there are a small group who have talent AND an interest in their contemporaries work, and perhaps use an MA to get recognised and get their poetry or fiction 'out there'. I've given it serious thought (though am sticking to academia methinks!) - the opportunity to sit around for a year writing poetry in the presence of poets who you've read before and respect does sound like a pretty good way to spend your time. But I would argue that £4,000 plus about £8,000 living expenses is an awfully expensive way of buying yourself contacts. There are other ways of doing it.