I believe the microjobs on mechanical turk are limited to US residents, and I don't think the microjobs themselves are country specific for this reason. Instead, they would simply be posted as they are without any distinction in country. The best microjobs to accept are the ones that require you to create content or transcribe audio and video. You can be paid upwards of $10 per job like this, but it takes more effort than a simple categorization task, which might pay out a few cents. I've heard of scammers posting a microjob and getting their worker account to get the first hit on it. Then, when others accept the job and do the work, they accept the work done from their worker account and get their money back. That's just what I've heard, though, so be cautious.
I just started on mturk less than two weeks ago. I kept seeing many people on forums and blogs using it to make $5-$10 a day, but I wasn't sure if it was worth it so it took me a while to give it a try. It is definitely a legitimate opportunity, but I wouldn't use it as a main source of income just another egg to have in your online money making basket. So far I have been doing a lot of short surveys and some other small tasks. There are some forums that give you really good advice on what to look out for when it comes to scammers. Also down the turkopticon add on to your browser. This allows you to see how others have reviewed the requestor so you can avoid scammers.