I've used it to advertise myself, so instead of a parent looking for a nanny, I did an advert as a nanny looking for a family.
Result was nothing. So that approach didn't work.
I am however outside of the M25 (by about 10 miles), so that could have been a factor.
Simply Childcare is probably worth trying if you live in London... but if you don't then I'd say it probably isn't worth it - though you never know you may strike lucky.
Most nannies I know will look for work at NannyJob , via local agencies, local newspaper, and on small ads at places such as newsagents, school, pre-school, toddler groups etc.
I wouldn't use Simply Childcare to look for my next nanny position - however, I am not in London, but in Surrey... If I were in London, then it would be something I would consider looking in.