i live in Marylebone and will comment on that area specifically as it's where i spend my days.
I love it as a member of a young couple. There are a lot of families that live in the area, however Marylebone is mostly flats. You don't see a lot of 'houses' until you start heading toward St John's Wood. Marylebone High street is a delight, with a ton of shops and restraurnts and boutoques. It is close to many tube stations (Baker St., Bond St., OXford Circus depending on where you live). We attend All Soul's Church and love it there, in Langham Place. Regents Park is a stones throw away (with the london zoo) which is very lively with the boats and things. Hyde Park is the other side of Oxford St. (busy shopping area) but a walk to Kensington Gardens or across Hyde Park is about half an hour.
It's a very safe neighborhood. When my husband travels, I have no qualms walking around at night by myself. Westminster (where Marylebone is) has one of the lowest council tax costs, which is nice (and amazing all at once). We also have a car and pay about £100 for the annual parking permit. Since we don't move the car during the week (only on ocassion) we don't pay a congestion charge.
I feel like I haven't spent enough time in the other areas to comment-only driven through them. I think I was expecting more of some of them.......but as i said, i haven't hung out there to comment!!
Also the Marylebone police are lovely and fast, we thought we had some people on our roof one night. They sent 2 cops over, they came in.....and seeing how our street is laid out (and the fact that we have a lot of construction going on around us) they sent out the Metro Helicopter! In pretty heavy winds!!!! They patrolled our street and areas, looking for any signs of bad things, but they either scared them off, or there was nothing there. Either way they were prompt and nice and we were very happy. There is a big sense of Community in Marylebone-people get together for the organic farmers market on sunday (behind the high street), i've met many people on the street, talking about not closing our fire station etc.
anyway, hope that helps