I haven't read any of the replies because I know this is a controversial subject and I don't want to get into a discussion. I had my DS at St John and Elizabeths hospital in St Johns Wood. I can honestly say it was the most incredible experience of my life.
Main advantages;
The time the midwives spent with you. I have never felt so cared for and looked after. From the moment we booked in until now 9 months later, they are still a part of my life and I am still in touch with them all.
I had a midwife with me from the moment I went in in labour and was never left on my own. I had tremendous support after the birth to help establish breastfeeing (including with a BF counsellor who worked one on one with me).
John and Lizzies places an emphasis on natural births and pioneered water birth in the UK. It is a wonderful, wonderful place.
There are other, more superficial advantages, like your own room, with ensuite and the fact that your partner is not just allowed but expected to stay with you after the baby is born. But for me the care is what makes all the difference.
Disadvantages; well the cost is all I can think of. No wait a minute, there isn't a SCBU at John and Lizzies but my thought process was that I would either give birth there, at a midwife led unit on the NHS or at home and none of these scenarios provided a SCBU.
If you don't want to go private, I'd encourage you to look at the new birth unit at St Marys which is also wonderful and very similar to John and Lizzies.
Best of luck and happy to answer any further questions.