Mturk is good for automating simple tasks that computers are bad at. I'm using it in one of my projects. My interest in it was originally for content generation, ie. Writing and rewriting articles, but I was not getting good results, probably because most workers on Mturk expect the tasks to be quick and easy. So if you think you're going to use it to generate millions of MFA articles and profit, think again.
What people are using Mturk: figuring out if an image is pornographic, dictating voice messages, transcribing business cards, filling out surveys, facebook likes, identifying businesses/people/things in images, finding details/leads about people or businesses - ie. Find their facebook page/phone number/email address, make an argument for a debate, etc.
Definitely worth a look at if you have a project that requires a human eye or ear. If you can automate the task with a computer, I'd say you're better off writing your own bot instead of using Mturk.
@TigerUK you can get around being in the US by registering a US corporation which is what I had to do. Bit of an investment though, so if you only use Mturk minimally it's probably not worth it.