I am legally S, but I live in the UK so things are different.
I went this way on the advice of my lawyer. Legal S over here deals with the split of marital assets and can also include details of the arrangements for children (ours does). It is legally binding for the financial matters but not on the kids - because ours are younger than 16 years, the court must approve arrangements and legal S does not go to court.
Why did I do it? Financial protection. At the time my WXW was all over the map and the very mention of D was a big drama for her. It would also have her running for the lawyers. So, I suggested that we agree between us the split and I would draft something up, get my lawyer to write it up properly and register it - she agreed and that is exactly what we did.
It seemed at the time to be a less confrontational approach than serving her with D papers etc, and I knew that would play better for me. WXW would have escalated things if she had an asshole lawyer at her side - that is for certain. It was probably a bit manipulative on my part but so what - she destroyed our M and in any case I did play fair with the settlement. I also paid the lawyer's fees.
So now I am legally S even though we have been living apart from almost two years.
I managed to agree a fair deal in the S process and its all paid up now - the house is mine and WXW got her share of the equity plus 50% of my pension. I pay my CS and no alimony.
We share the kids probably around 55/45 (in her favour) but it works OK and we're as co-operative as we can be at this stage.
Also, the agreement that we have paves the way for a quicker, easier and cheaper divorce.
So - I think it worked for me. I don't know if it is cheaper - my lawyers bill for the legal S was around £2,000 plus about £700 for the legals for the house transfer and a bit extra for a new will. All in - £3-3,500 in fees and charges.
If I raise D (and its uncontested, which it should be) that would be another £1,400 but I hope to persuade WXW to do this. I'm well past the stage of wanting to be the one to file and would much rather save the cash. Her mid life crisis has cost me quite enough now thanks.
Hope this helps