It's unbelievable that no one has swooped in to crush MTurk given how little Amazon has done to develop it since 2005(!)
I've used it a few times and because the eco system is truly about hiring dirt, dirt cheap labor you kind of get what you pay for.
Beyond that the revenue model sucks - the concept of running millions of micro-transactions and having a little piece of all the action is compelling, but again because the product use is so limited it's difficult to make signifigant revenue.
If the purpose is dirt-cheap labor, they should open up the door to markets where $cents matter.
At my last job I was involved in a project where we were going to use CrowdFlower, but it ended up being canceled. I was always curious about how good their results are.