I have one, hella fun! I bought it from JCWhitney last summer. The $50 one with like 36 sounds. I mounted it between the center console and driver seat. It's more for entertainment purposes. It works really good close up with the mic, but if someone its like 20+ feet away, they won't be able to hear you, it'll be all muffled. But the sounds, sirens, and animal sounds rock! I use it all the time for fun. I used it today while taking a friend home, some guy was crossing in the middle of the street, and I said "sir, maybe you want to try crossing at a crosswalk next time." My friend was rolling. It's also fun when driving around doing nothing with a bunch of friends. Animal sounds too! Especially the cow! Fat lady walking by, "Moo!!!!" Also the ice cream truck music. I play it when rollin by the elementary school. Gets them everytime!

But if you wanna really be able to yell at people, then I'd suggest investing $300 into a speaker ($150) and $150 into the siren. Here are some examples from Galls.com

Siren:
http://www.galls.com/shop/viewProduc...jsp?item=SK079

Speaker:
http://www.galls.com/shop/viewProduc...76&catcode=121