I can let you know my brother's experience on the RSMS visa before July 2012 changes so hope it may help a little, although things have changed so take it with a pinch of salt.
He was already in Australia but his employer was not a registered sponsor. I think it does make the process easier and quicker if you can find an employer that is already a registered sponsor, however it's a very quick part of the process and I think it only took a few weeks (maximum a month) for my brother's employer sponsorship registration to be approved so it's not a big hassle.
My brother had only worked for them for a few months before they started the visa process. From start to finish including a bit of a delay in getting PCCs couriered and sorted out, it was 4 1/2 months in total from start to grant. I know the 187 RSMS visa has highest processing priority and the 186 ENS visa has second highest priority. I can't remember now if he did a skills assessment, but I know he did have to give a certified copy of his degree, not sure if it was for an formal skills assessment or just for DIAC. But I know with the current 186 and 187 visas you are only exempt if you've worked for the employer for 2 years.
There were some very strict rules and regulations. His employer had to sign a 2-year contract stating his nominated occupation and the job description had to match that of the Australian description for that occupation. They also had to agree to the market salary rate and show proof of how they came to that salary. The employer will also very likely be required to provide proof that they have advertised the position and interviewed Australians for the position to show that they are giving Australians a fair chance, however I'm sure Stephen will help you with how your potential employer would go about this. And often if the position is being advertised before you applied for it, chances are they can prove it easily, whereas in my brother's case they had to then post adverts for the job because he was already working for them and they knew they would prefer to hire him because he knew the job, but they had to advertise it and interview other people anyway.
My brother is officially free from him obligations this month after 2 years since his visa was granted and says that it has not been too frustrating or stressful being employer sponsored, however he has a potential promotion coming up, which means he may even stay with his employer from here forward. He is in Margaret River, Western Australia and loves living there. In ACT, Western Australia, South Australia and Tasmania the whole state is now considered regional so those are great states to start with if you don't want to be limited to an area. But the other states like Melbourne, it mostly just excludes the greater city centre area.
I am sure that if my brother knew that state sponsorship was an option back then and if his occupation was available which I actually think it was, then he would have gone a different route, just because everyone prefers independent or state sponsored visas because there is less risk and you are not bound by employment. But if it's your only option, at least it is a PR visa and you just have to stick it out for 2 years and then you're a resident and can do as you please and go where you please.