Did Edinburgh this year and hope/expect to do it next year as well.
There was a problem with the distribution of water during the event. They didn't have enough water in the right places at the right times, but hopefully this will be rectified for the next race. (there were reports of water thefts in the hours before the race)
Overall I had no problems with the pre race organisation or the start (apart from the announcer who thought a gloriously sunny day was a good day to run a marathon!). I found no congestion issues during the race or at the finish. However, I will add that I was in the leading 10% of the field and probably clear of the areas before congestion occurred.
If they are adding a 1/2marathon on the same day they will have to stagger the start times (I'm assuming). But to have double the numbers finishing in Musselburgh there will be a lot of congestion, both in the race and afterwards.
I would reccomend it as a race to do but would probably advise waiting to see if there are any surprises with running a marathon and a half marathon at the same time.