I had one when I had dd2 - absolutely fabulous, and I would do so again without hesitation.
dd1 was 2.6 when dd2 was born, and I was in all liklihood going to have another caesarian (I did in the end). dd1 has severe ASD (just going through dx att he time), and our lives were a hectic whirl of appts, keeping routines etc.
we wanted her to have as stress free an intro to her new sibling as possible, which for her meant making sure she could get to eg Tumble Tots, nursery, toddler groups etc as usual.
I bf dd2 as well - perfectly possible with a maternity nurse. we shared care in the day )of dd1 as well as dd2 - nanny drove everywhere obv, as I couldn't), and then of an evening, she kept an eye on dd2 while I ate dinner with dh, brought her to me whenever she needed feeding, and I handed ber back afterwards for winding, endless pacing up and down etc. lovely.
for the first few days - you know, where they don't settle at all and you just sleep (or not!) with them snuggled up, I had her all night. but after that, she stayed with the nanny until she woke for a feed in the middle of the night, then I fed her, and usually curled up in the armchair in her room until morning (dd2's room, not nanny's 
). post early morning feed, nanny had her again, while I had a bath, sorted out dd1 etc etc.
it meant I got at least half a night's sleep, and combining that with eg an afternoon nap while dd2 was quiet was a godsend.
we had the nanny for 6 weeks, and it was fantastic.
we had no posh facilities - just a spare room, really, and nanny shared our (single) bathroom.
dd2 is 4.6 now, and has suffered no ill effects from being handed over to a stranger 