Ease of Use: 4/5
mTurk favors the worker. There is no rating/feedback system; when you submit a task, it is either approved and paid, or rejected and not paid. At first this may seem like a bad thing, especially if you are just starting out on mTurk. But believe me when I say this is actually a very, very good thing that absolutely works in your favor.
Why? Because the power is in the hands of the worker. On eBay, a few bad ratings can tank your sales. On oDesk, unless you have really impressive feedback, you won't get hired. However, on mTurk as long as you follow the instructions for the task, it will be approved and you'll be paid.
Additionally, there is nothing barring you from working on mTurk. You can accept any task you are qualified for, and on most tasks, the only qualification is a high approval rating. No sweat.
Payment. You get paid for the tasks you complete on your Amazon Payments account, which you can use to shop Amazon and a bunch of other sites: https://payments.amazon.com/sdui/sdui/personal/directory
If you are in the US you can also have the balance transferred to your [verified] bank account.
This really blows because while the payment is quick, you can only use the money towards purchases on Amazon.com or their affiliates unless you live in the US. If you are not in the US: unless you purchase from Amazon a lot of the time, or you're specifically saving up for a big purchase (again, on Amazon), it's pretty much not worth it.
The only redeeming factor here is that payment is quick, usually 1-3 days from your tasks being accepted.
The risk here is negligible. You're not investing anything or giving out your information to 3rd parties, and payment is pretty much guaranteed.
Who would I recommend mTurk to?
-Live in the US & have a US bank account
-Already have specific things you buy or want to buy from Amazon
-Are bored, have some time to kill, and don't mind doing quick mind-numbing tasks
Then mTurk is probably for you. Can you make a living working on Amazon Mechanical Turk? Maybe. If you're willing to look at it like a real job, work several hours a day without many distractions, and are in the US, you could probably pull at least $3-400 a month. Honestly, though, there are a lot of other websites and projects I would recommend if you are looking to work and support yourself exclusively online.
mTurk is a nice supplemental income. I think in the last few months I've made about $60 and I did it for about an hour a day or less.