Pros:

+ Ponies (specifically, Rainbow Dash)!
+ It has a cutie mark! (Then again, all the weapon skins do.)
+ It's a guitar! (But you knew that already)

Cons:

- The frets seem too colourful for me, but I'm sure I'll get used to it.
- No picture.

Conclusion:

A worthy addition to my steadily-growing collection of pony weapon skins.

Pros:

+ Excellent remake of Half-Life 2: Episode 1 from the start of Lowlife to the end
+ Half-Life 2 references such as hidden headcrab zombies in every map, a couple lambdas, and a few dead Combine soldiers.

Cons:

None that I can think of.

Conclusion:

A must-have for the Half-Life 2 fan trapped in the zombie apocalypse. Brilliantly re-designed levels from Half-Life 2: Episode 1 give the campaign a familiar feel, yet create an entirely different experience. The campaign is challenging enough that completing gives a sense of accomplishent, but players don't feel overwhelmed during gameplay.
Also, music from Half-Life 2 plays at certain points; whether this be an oversight by the creator during map re-creation or a clever inclusion of ambiance and reference to the campaign's orgin is unknown to me, but I just think of it as a bonus.

Pros:

+Original concept
+Unique finale idea
+New survival content
+Half-Life 2 references

Cons:

-No tank music. 
-No preparation time for the finale.
-Finale can be confusing to those who haven't played L4D2.
-Bots don't know how to avoid a moving train.
-Bots don't take weapons and medkits at the starting area.

Conclusion:

While this is an interesting mod, especially for those who have played HL2, numerous bugs make it a less-than-enjoyable experience. Consider downloading for a use or two if you have HL2, but otherwise skip it.