Station 51 is a single player mod for Half-Life 2: Episode 2. You are on the run from the Combine closing in on your outpost. Can you escape?
The 6 is mostly thanks to the level design. It was very pretty, fun, and the battles were well placed. The maps seemed to go on forever compared to most Source maps (I got about an hour of of the two maps), which is a good thing. The detail was nice, and I always felt pretty perfectly supplied, health and ammo-wise. That's mostly where it ends, unfortunately.
The mod lacked background music (or sound at all) as far as I could tell, there weren't any puzzles (just finding buttons) and the areas that the bigger battles took place in were somewhat arena-like. The finale was also the worst part of the mod in my opinion. Very anti climactic, and the areas preceding it lacked the detail that the rest of the mod did so well.
There was also no story, which was disappointing. As soon as the mod starts you get a message on a radio telling you the combine are on their way, but that's about it. This, combined with the lack of any puzzles made the game feel like an hour long shooting session.
If you're looking for a pretty, mindeless shoot 'em up, definitely give Station 51 a go. But if you're looking for something featuring puzzles and story, you'd best look elsewhere.