Is The Hidden Ones DLC worth getting?
If you liked the main game, yes. There are three new areas, and while they're not huge, they are big enough to get lost in, and both the geography and the cities feel new, not just retreads of places you've already seen on the mainland. There are story missions, sidequests, and stormable fortresses; and even new papyrus riddles and stone circles. Also, the Sinai just has a lot of things going on- unmarked rebel outposts and cannily hidden treasures. Those details don't affect the main quests or achievements, but it is place with lots of nooks crannies to explore, even after you've finished everything else.
The raised level cap and additional equipment slots don't completely unbalance combat, but they do provide a nice sense of progression, and having 10-11 predator arrows to call on at any given time makes snipers even more snipery than before. The special mounts and weapons you acquire here aren't much more powerful than ones you already have, but they are fun.
One downside is that the DLC doesn't really advance the main Assassin's Creed story arc much, but again, if you liked the AC:O maingame, this is probably a tradeoff you can live with. A little more disappointing is the complete lack of any First Civilization ruins or encounters, but that doesn't really detract from the things that are here.
One tip- for the moment it looks like the combined cost of the story DLC is going to be less than the Season Pass. If you don't want the extra loot that comes with the Pass (mainly costumes, with some Final Fantasy gear), you can save some cash by just purchasing the story DLC on its own.