I applied for sitters, didn't end up working for them but I was told a lot about how they work by their local rep. You can't pick and choose your jobs, my area had a text alert and you had to respond to every alert within a certain amount of time and you could only turn down a certain amount out of every ten you were contacted about. Not sure if she was trying to keep a well oiled ship or if you'd actually get dropped from the books but it was helpful to know that before I'd signed up! It wouldn't have worked for me in the long run. It was a popular service in my area and I was told to expect at least one job a week, possibly more. They seem to go through sitters quickly here and it doesn't surprise me.
I live in an expensive SE city but it only qualified for the lesser rate, so a three hour booking wouldn't be worth anything once you'd paid for parking or a bus to get to the job (think it was £6.50ph). I know that the area had an issue with people booking longer slots but only being out for a few hours just to guarantee a sitter taking the job. You wouldn't be paid the full booking even if they did return early.
Families can ask for you specifically but they will still share the message with everybody else in the books so if you don't respond first you will lose out on it.
My employers use sitters when I can't babysit or one of their others cannot sit, they have mostly been happy but they get annoyed by the random sitters being sent sometimes, on a few occasions they've contacted their preferred sitter first to check that she's available but when they've tried to book her through sitters (as they aren't supposed to book her directly) she's never been sent the text and they've been told that she's unavailable.
I'm relocating later in the year and am planning to sign back up to it then as the new area is much cheaper to live in and get around, but if you live in an expensive area or an area plagued by parking restrictions or bad traffic I'd think very carefully about whether it's worth it.