I have actually put in about 30 hours or so on the mturk website as a personal experiment to see how the system worked, whether it could be profitable to do it for a living, etc. The answer I found was largely no (i earned about $35 dollars so almost exactly $1 per hour), however there are a lot of problems with the mturk site that if they were resolved I thought it could be much more effective.
One of the most obvious things is that they offer "qualifications" so that you can show you are likely going to the tasks correctly and thus ask for higher pay (since the job submitter doesn't have to give the task out 5 times to make sure its done right). The problem is that the qualifications are not standardized and there is no way to search for tasks that you are qualified for (you can hide tasks for which you are unqualified but there is no way to show _only_ tasks that require qualifications that you have).
This makes it harder for skilled workers to find jobs (thus driving them out of the market) and it leaves only the people who do the tasks poorly because they don't have the proper skills, or the people who are willing to work for a few cents per hour. This is apparently a well documented economics phenomena called a "lemon market" and there is a pretty good blog about its relation to mturk here: http://behind-the-enemy-lines.blogspot.com/2010/07/mechanical-turk-low-wages-and-market.html
I think the mturk idea was a good one, it is just that amazon doesn't seem to interested in developing it (I signed up something like a year a go and I have yet to see a single change to any aspect of the website). I think something like this is naturally suited to bitcoins and would really be a boon to the bitcoin community as people could earn bitcoins by actually doing something rather than just mining (nothing wrong with mining, I do it myself, but eventually the bitcoin economy actually has to produce something).
i also did a similar thing, working for about 5 hours for 15 dollars. there were very very few skilled tasks available, like one at most offering dirt poor wages. i mostly wrote articles and found the people were reselling off the articles i wrote, these were journalists for poor magazines. i found this whole community of people mturking for fun, like this one woman it took her 2 hours to transcribe a 10 minute video and she was old and just doing it for fun, like farmville or something. the thing is, these people were doing the worst of the worst possible jobs, repetitive mindless task, very few people would put in offers for the ones which required minimal skill level like 'write an article on daycare'.
the biggest problem with mturk is that to advertise jobs is difficult and requires you to sign in blood. because then there were so few people offering work and so many people wanting work, the employers were scamming the employee's. to resolve this issue the system needs to be more fluid.
another problem is that you can only cash out if you are a us cardholder, when largely the people willing to mturk are from poorer countries, but for some reason they were doing it anyways? the employers are kind of aware that this is poor people on this website and are exploiting the users because they know they can.