Pros:

Custom textures
Layout
Great replay value
Atmosphere
Lighting

Cons:

To easy
Ripped models from Portal (I'm really against this kind of stuff)
The weapon room
Kind of difficult to find the radio (took me 2 mins)
UFO could be better looking (would be even better if you could go in it)

Conclusion:

Great map! Really had fun playing it the voice over was really good.
Suggestions:
Take out a lot of the guns in the weapon room and spread them out over the level
Make GLaDOS activate the hoard :30-1:00 after start that way survivors can't get geared up fully
I know the Portal models look good but you really shouldn't be using them