I have this PA/siren in my S-10 Blazer: (well, not this exact one, but similar, just an older model)
When I bought it and installed it, it was mostly a novel thing that I thought would be cool to mess with my friends with.
In theory, it was great. However, in practice all I really use it for is the "air horn" that's part of it and occasionally telling someone at night that their headlights are not on, that someone should turn off their blinker, or moo'ing at cows when I drive past a pasture (yes, I've actually done that one, it's kind of funny).
On a scale of 1 to 10 on "worth it" investment, I'd give it about a 4. It's kind of cool, but after a while it's nothing special and you end up wondering why you spent so much money on something that you never really use.
And if you ever click it over to one of the siren noises and actually use it on public roadways, you can easily get yourself in a LOT of trouble very quickly. I've been able to show my restraint, but it's incredibly tempting to use it for irresponsible acts and get yourself in a world of legal hurt real quick before you even realize it.
Long story short: save the money and just upgrade your horn to something a little louder, because that's pretty much all you'll ever use it for.
