It was nice to have a brief compilation of their school days, some of their present-day, and final future interviews. It's a 35-minute single episode. There are no cut scenes or new content, sadly.
But it's nice to have simply their school scenes, some of their interviews, constant bickering, and Yeon-Su's nagging. It was really fun to watch. If you liked OBS, you might like The Documentary too.
The Documentary is available on SBS's Official Facebook page and is in Korean only. If you watched the series or enough k-dramas, they would be unnecessary.