anabella - I'm one of those.
I moved to London as a student and thought I'd live there forever. Now I have 2 DCs we have just left and I've moved back to my hometown.
I miss London - the parks, the museums, the decent restaurants. But on the whole I am happy we moved.
If I could have the life I have now, but in London, then I would have loved to stay but we couldn't afford it. Our house was small and our garden tiny. We lived in an expensive part of town but couldn't keep up with everybody else. We couldn't afford the school fees and the state schools were poor (and I couldn't be bothered with the scrum). The nursery fees were so high I would have had very little left if I went back to work. We didn't have the time or the money to do all of the things we loved before we had children (dinner, theatre, long afternoons in the pub).
I'm sure we'll miss it as the children get older but we are only 50 minutes away now and I actually visit central london more now than I did living in zone 3!
One thing I have really noticed is the 'buzzing cosmopolitan' thing. Actually, although London is diverse in terms of nationalities, the area I used to live in was populated by people exactly like me who read the same papers, wore the same clothes, had the same buggy, same changing bag etc. Now I'm surrounded by a huge wealth of people from all different paths ... far more so than I ever did in London.
But I can't get a decent organic burger for love nor money