Here is how it works for open seating as I just got off the Ryndam and as it was a last minute booking I was given the open seating.
On the lower level ( deck 7) there are two podiums set up with a computer and a DR host at each. The left side will be for those who are just dropping in to eat and the right side is for those who made a reservation ( if you want to eat at a particular time you must call ext 88 and make a reservation for the open seating every day ). They will ask your room number and if you are willing to sit with others. They now ask your room number in each dining venue so they can keep track where folks are choosing to dine.
When DH and I ate at a table for two we were served and finished eating in about 45 min with this new policy. When he ate in the Lido and I dined with others ( usually 2 couples ) it would take the normal 1.5 to 2 hours.
I do prefer the traditional dining. I can see where it might be a problem for those traveling solo to not get stuck with couples all the time at the table.