My main problem with this (besides the zombies) is that they get weird about the whole"five years later" thing at times. Towards the end, Shepherd comments "No one knows who these guys are or where they came from." If it's 5 years later, you'd think they'd have some idea. They reference City 17, so the Combine administration should have started right? Otherwise, the city would still be called Odessa (that's my theory for what the city actually is, anyway). So how can they have no idea who the Combine are? Even if all they knew was the "Universal Union" BS Breen was pushing, that still seems to be more than they knew here. Also, they're fighting Synths! As in, humans turned into Combine. They should know more, at this point.
They're part of a larger resistance, so how can they not know things regular civilians would?
Nitpicking aside, it was quite good. I prefer Escape from City 17 because, among other things, the setting itself is more half life-y (tell me that train yard didn't feel like City 17). Really, there was nothing connecting this to the Half Life universe besides Shepherd's name, the sound effects, and the Combine masks and strider, all of which (besides masks) were added post-production and actually add nothing to the story. They should have made this stand-alone.