When you first play it you're just amazed at how large and nicely modeled it looks.
One thing that was surprising is that both times I played on it, the marines won. So it does seem marine sided, despite the fact there are vents everywhere. Since hallways are long and some rather large, marines have an advantage of being able to attack from a fairly long range. Also, it seems the ms is pretty easy to defend, since there are two main entrances: the door, the tunnel under the cc. The only other entrance is by pipes which are jumbled together and are somewhat hard to get over as an alien (esp. an onos).
I think it needs some refining, since some of the vents are confusing, and it's harder for aliens to defend places early on (and we never actually got far enough when the aliens could). Once everyone gets a little more used to it, I think it will go somewhat more smoothly.
EDIT: For making sense.
Edit: one = won :pEDIT: You edited my post...
. Being all sneaky and :ph34r:-like.