tl; dr - Buy this only for multiplayer/OST. Campaign is awful.
I waited for pretty long time to get this DLC (till it gets 75% on Steam), and when it finally happened I bought The Forgotten. I regret buying.
I felt like its going to be amazing since they released an expansion with additional campaign for remake of old game. I thought it should be a fresh breath of air for old game. I downloaded The Forgotten for one night, and on the next morning when I launched The Forgotten I became... angry. This is impossible to explain my whole feeling since it was just one big dissapointment. Developers just made several scenarios and combined them into campaign. You will not notice speech in intros, no voice in scripts ingame, no action, and you will regret buying this DLC at all. Some people can say its related to fact that originally this expansion was developed by community, but I don't this its necessary to be mentioned when it was released as "official expansion". If it's released as Official, then I'm waiting for quality, not just collection of scenarios.
OST is amazing with exception that its pretty small. Environment is pretty meh, but on "satisfactory" level. Campaign's maps are full of bugs. Your units are getting stuck because of bushes on the way, some maps even include very bad positioning of barracks where your units stuck somewhere between buildings right after hiring. Scripts that move your camera to show attention sometimes do not start at all so you keep forgetting where you're supposed to go with your character.