Stellar campaign, Valve quality.
Simply one of the best custom campaigns out there - if you have any interest in playing custom maps at all, this is one you should have. Well thought out and executed map design, a degree of interactivity and storytelling that goes a step beyond, without becoming too much. A must have vanilla-y experience, with just enough quirks to make it memorable and unique. Supplies are adequate, gameplay is fun and overall well balanced. Good solo with bots or with friends. Even after half a dozen playthroughs, I still come back to it. My only minor gripe is that you spend a fair bit of time without a primary weapon in the first map, which is frankly something done to death by custom campaigns.
Great up until map 4.