The trusting someone you haven't met is a potential problem, though I expect parents with older children are more likely to go with it as the children can say if they didn't like the person.
Once you have had someone, you can request them in future... if they are available. You may end up having a few different local people who you book via Sitters.
If you use a babysitter quite frequently, they can be a lower cost option then using a local childcarer directly. If you don't go out very often, then it can cost you more.
For example... babysitting 3 Hours per evening, for 6 evenings (not a Satuday night) in a one year period (in GU postcode area)
Sitters would charge:
Minimum Fee (4 Hours x £5.80 per hour) £23.20 multiplied by the number of evenings out (6) giving us £139.20
They then add the Booking Fee (£ 4) multiplied by the number of evenings out (6) giving us £ 24
Then we add the Membership Fee (£ 12.75 per quarter) thus £51 for a year.
This gives us the following calculation: £139.20 plus £24 plus £51 which gives us a grand total of £214.20
In comparison, for evening babysitting I would charge 3 hours @ £8 per hour (£ 24) multiplied by the number of evenings out (6) which gives us a grand total of £144
Why I have used 3 hours in the calculation? Because I have found that parents I typically babysit for only go out for 3 hours, unless they are going into London. So to get most value from Sitters... try to be out as close to the 4 hour minimum period as possible.
Try it for a while, see if you and your children like it. You may find that in your area you have great people doing the babysitting... or you may find your children don't settle with them.
How old are your children? Will they be awake when the sitter arrives... if so, then they could interact with them for a while before going to bed.