Pros:

- it lightens up the apocalypes
- it's pretty funky
- there's nothing like killing zombies with rainbows (LOL)
- it takes you away from all the seriousnes of l4d and lets you have some fun

Cons:

- doesn't really fit l4d

Conclusion:

Really good modeling. I'd recommend it to people that like rainbows I geuss lol.
It would be like 50X better if you could make it shoot rainbows. THAT would be funny