We have been here in Sydney 9 months now. As the others said, it depends on the salary, and also on the visa you will be moving over here on.
We are here on a temporary visa (a 457), which means that we are not really regarded the same as citizens or permanent residents, and so the answers to some of the questions may not be true depending on what visa you would have. We are also only here for 2 years, rather than forever.
a) I would be lying to say we had no regrets - I really miss being able to visit family easily, which is probably made worse by it being the run up to Christmas. However, with skype and cheap phone calls, it's nowhere near as bad as it could be. Other than that, we love it here - if I could move all my family and friends out here, we wouldn't ever go home. It's a really nice city, and if you choose your location carefully, the transport isn't too bad either. There's loads to do and see, which means that you can be really busy if you want to be! It is very expensive - both to rent and to buy (there's no way on Earth we could afford to buy in the city),but it depends a bit on where you are used to living in the UK - if you have been living in London, the house prices probably wouldn't seem too awful. General cost of living (for food etc) is quite high, but eating out and petrol are cheaper.
I wish we had know how ultra competitve the rental market really is - findign somewhere to live was a bit of a nightmare, and you really do have to compromise if you want to find somewhere within a reasonable time. It's also quite strange being so far from anywhere that isn't Australia - I know that a lot of people find it hard that they can't just pop on a plane for a few hours and end up somewhere very different.
b) We are entitled to reciprocal medicare because of our visa, which means that any immediate or essential healthcare is covered. Because of this, my current pregnancy has been free through the public health care system, and so far, I can't fault them at all - they have been brilliant despite some complications with the pregnancy. When we go to the GP, it's $60 an appointment, and we can claim $30 of that back through medicare. If you aren't entitled to reciprocal medicare, then whether or not you get medicare cover depends (as far as I know) on how much you earn, and most people seem to have private cover, which I know very little about (we have private cover, but it's not Australian, os will be different)
c)I have found it a bit slow to make new friends - it's getting better now, and I am meeting people more through playgroups etc, but it has taken a long time. Having said that, I am quite a shy and quiet person - I think if you were more outgoing, it wouldn't be so hard.