If you're picked a good department, they *should* give you a supervisor who is reasonably well-qualified in your chosen topic. I got a brilliant one two times in a row because of a (worrying) obsession with Greek tragedy. If you have a vague interest in a topic but suspect that the departmental expert on it will be a fail of a supervisor, avoid it like the plague. And a good department should be willing to listen to your concerns about supervisors as well. Talk to previous postgrads if possible.

Mine was a great experience, overall. However, a) I'm in the UK and the fees were much cheaper in 2008 and b) the above has more 'shoulds' than you might be comfortable with...