I used to use them occasionally when I was a single mom. I had tons of medical debt, no credit cards, and terrible credit, so it was the only kind of credit I could get. I made too much income to qualify for any social services or charity, but my income could vary from $1000 a month to $2500 a month, and sometimes the lower months would happen two or three in a row. I saved as much as I could during the higher earning months, but invariably I would find myself out of food with no money left in savings. Since my income was largely seasonal I knew when I could count on business, and my income, picking back up, so I never had a problem paying them off. I also was careful to not borrow more than I needed.
How it worked was this. For every $100 I borrowed the fee was $15. I wrote a check for $115 (if I was borrowing 100) and post dated it. I could choose any day within six weeks to post date it for. On that date my check would clear. Yes, it's expensive credit, but it saved my butt on many occasions. Sometimes if I got sick or missed a lot of work I would go through my savings too quickly. Sometimes the fee to borrow a couple hundred dollars was less than the $150 fee for paying rent late. If I didn't have money for bus fare I'd have lost a lot more than $15 by not being able to get to work.
I agree with CPA-Kim. I think they should be tightly regulated and have more consumer protections, but I don't think they should be illegal. When people have no other options they will have no choice but to turn to illegal loan sharks, or other illegal/morally questionable activities to get the money they need.