Hi! I've looked into NannySOS before and from what I've gathered, they're a MOM-licensed agency that's mainly known for confinement nannies but also covers babysitting and longer-term arrangements, so quite versatile depending on what you need. One mum I know had a confinement nanny called Sew Lan from them and said she was really attentive, took care of both baby and mum well, which is exactly what you want during that exhausting period. The general sense I get is that the process is fairly fuss-free and their nannies are experienced enough to communicate clearly about the baby's needs, which helps a lot when you're anxious and still figuring things out.
That said, as with any agency, I'd still suggest doing a proper interview with whoever they assign before confirming, just to make sure the fit is right for your family.