In the major hubs, you do get perks as priority lanes. Usually that's it. Many KLM/AF flights (intra-EU) don't have a priority boarding anymore. For INTL trips, the priority boarding is either a hit or miss. On AF, they let ST Elites and Elite Plus board in zone 8 after their own economy. But sometimes you get an extra call for priority boarding - inconsistent to say at least.

On AF you have access to preferred seating with DL Gold or higher. On KLM you have access to the seat maps on some flights but they reserve the right to charge extra you for those seats - in particular the bulkhead/exit row seats. Short intra-european shuttle flights have no seat selection prior to 24/30hrs of departure, all seats are assigned at check-in. I have no clue why I can select them for free sometimes (as DL Gold) and why they want me to pay sometimes - e.g. my outbound is free of charge and the very same seat on the inbound in the same booking class and reservation is 100€

Be also careful with your luggage, on AF/KLM flights with no DL code share you are not allowed to bring two checked-bags and you are not granted the extra seven pounds. Strct rule of 1 piece of checked luggage in Y with a maximum of 23kg. Anything else and you will be charged - I have seen them many times to strictly enforce this rule