The content covered varies from covering generic fantasy races(e.g., #704 Dwarves, #711 Elves, #707 Dark Folk) to full adventures. The adventures are generally well written with detailed maps and nice drawings to add to the atmosphere. The adventures are designed for use with Basic D&D, AD&D, or Tunnels & Trolls, varying with each adventure but AD&D seems to be a constant. I'd imagine that porting them to a newer system wouldn't be very difficult.
The themes covered varies from fantasy-medieval to historical. Quite a few books deal with Romans, Celts, and so on. #717, Shadows of Evil, sees the Romans portrayed as the evil invaders of Celtic land which is not something you see very often. Shadows of Evil contains a good deal of real historical matter and there is little apologia here with regards to certain forms of pagan conduct. The adventure in #717 itself is fantastical with semi-historical elements.
As stated before, the maps are well detailed and so I will provide an example(again, from #717):
They also provide you with various things such as crudely drawn maps(as drawn in-universe) to show the player:
#717
Shadows of Evil is a lengthy adventure, coming in at about 90 pages when counting the maps.
There are also books like #719
Fantastic Treasures that are just a collection of items(or item ideas) to use in your campaign. #719 itself is interesting because it involves many historical & mythological artifacts.
It's not very often you find adventures with the map presented in 3D(From #720
Fez 3 Angry Wizard):
The books focusing on a single topic are, for lack of a better word of praise, very well done(#721
Dragons)
Again, they covered a wide range of singular topics from Dwarves to Giants, Lizardmen, and so on...
The quality control would be of what I'd deem "high", very little typos or obvious errors. If I were looking for fantastical and/or semi-historical supplemental material, this would be a great resource. These were definitely high enough quality to have been published by TSR themself. I don't have much else to add about these beyond praise. I may give one or two individual reviews, but I don't think they really need it because if you just pick one at random the quality is high.