We had a young lesbian living with us for about almost two years. We did not go through an agency. She was a student. It worked out well, for the most part. She was 19 when she moved in and didn't really want to be on her own yet. Her family paid us 400 a month for her to stay with us (the space she had could prob be rented for at least 700). She would be there when kids got home from school, take them where they need to go after school etc. Basically just take care of them from like 4-6:30 3 or 4 days a week. And if she didn't have plans she'd babysit at night if we wanted to go out. She also pitched in with the pets and stayed with them for the entire weekend occasionally if we had to go out of town. Kids were 8 and 10 then, so they needed less and less supervision as time went on.
It was pretty freaking fantastic actually. She helped us out a ton and paid us 400 month to do it! Actually her grandmother sent us the check, but no matter.
There were downsides. She was much closer to their age than ours and I would forget that. 19-21 year olds can be very emotionally immature. Her lesbian relationship drama got to be very tiresome. She had no dad growing up and a lot of those insecure feelings about her dad got projected on to me sometimes. And she had two cats that I couldn't stand. Mind you, we already have 3 dogs and a cat so our house is basicallly a zoo anyway. But god I hated those two cats.
Edit - It wasn't just take take take for us. We did a lot for that girl. I pretty much spoiled her a lot of the time like she was my own kid. We were kind of a surrogate family for her. But alas, all birdies must leave the nest at some point and eventually it was time for her to go out on her own.