I love the game, and I bought all it's DLCs, but this one is generally the weakest.
You should concider buying it, based on your general game progress.
If you're looking for the additional end game experience - then avoid it, since it's only a set of maps, that leads you to a new treasures.
If you're extremely well equipped - then avoid it - you won't find anything better in the treasures.
If you're somewhere in the middle of the story, then go ahead and buy it if you want to find the end game equipment much earlier.
The general feeling is that unless you want to have Henry equipped in Warhorse logos, then you'll easily find as good equipment, for free, in the game.
BTW. Hans' Capon sword, that you can steal from the chest in one of the quests involving his bath, is the most powerful sword in the game, that I was able to find), and even the Treasures of the Past don't change that.