Pros:

*Great environment, each map has a unique feel to it
*Great flow from map to map, fairly well paced
*Well optimized, except for a few laggy areas (particularly where there's water)
*Nicely tuned for versus play
*Great attention to detail for a beta
*Easily the best desert-themed campaign I've ever played

Cons:

The color correction is a bit too extreme.  I'm guessing you're going for a sepia tone sort of thing but you need to turn it down a bit as it is a bit glaring (the sand is so bright its almost white), it makes me feel like I'm walking on Venus.

A few maps are a little bit long for versus, and I think map two (I think) might have some nav flow issues... It scores points inconsistently (giving more points to the team that made it less far).  You may want to double check to make sure that the manhole cover that you open up from later in the level blocks the nav so that it doesn't make that ladder part of the escape route path.  There might be something else at play there, not really sure.

You might want to consider adding some sections of some of the maps where tanks aren't allowed, as there are some really really tight areas where its just incredibly difficult to fight a tank, especially in versus.  (Contrary to what some other reviewers have said, I encountered nearly a tank every map.)

A couple ladders could be added on the sides of the sandy hills, as its difficult to find spawns in some of those sections.  I know ladders going up displacements is kind of a pain but it's doable.

The radio audio on the finale is a bit overly scratchy sounding, and I had to turn down my volume a bit.  I'd turn down the noise effect a bit on it.  Also, the finale, as well as a few of the other maps, could use more melee weapon spawns.

While most of your map was pretty well detailed, a lot of the walls along the military base were kind of blank looking and textured all with the same grey-ish color.  Otherwise the level of detail you have is great.

Conclusion:

Great campaign!  This has become a new favorite for versus among my group of friends.  With a couple bug fixes and toning down the color correction, this map will be perfect!