I have used it. I passed all the exams (I did UK En, US En and Aus En) and i can't remember which one was 3 separate tests, but it took bloody ages! I also bought a foot pedal which helped, but honestly, the pay for the amount of work it was was terrible. I still have $20 that I earned sat there because you have to earn over a certain amount before you can withdraw it be I got fed up.with it, so have never actually withdrawn my earnings.
The other thing to consider is that when you press the button for work, it will give you a random 3 minute section which could be from the middle of a conversation so you have no context whatsoever. Sometimes the accents are so strong or the audio is so bad it's really hard to get what they're saying. There are a lot of names etc that are really hard to get. To start with, I was trying to all the excerpts I was sent, but I realised it was mainly the really shit ones that took over an hour for 3 mins of audio, which I think other people were rejecting. You can only reject a certain number before they make you wait a certain amount of time (an hour maybe? I can't remember). And they can reject your work really easily if they're not satisfied. So you've spent an hour for nothing.
All in all, I think there are better transcription jobs out there, but if you're not too fussed for the money initially, it could be good practice. Def get a pedal though (although it will probably take you 2 weeks to earn enough to pay for the pedal
).
Hope that made some sense, happy to.answer more questions if you have them.