I had one on my vehicle about 9 years ago, before they were illegal to have. It was lots of fun... just not when you accidentally set off your car alarm. Anyhow, if you install one, I'd recommend adding a switch so you can go back and forth from your stock horn to that one. That way if you get pulled over for it, you can switch back to your factory horn and try to deny it. Also, make sure you get a large tank because those horns will move air pretty fast... you only get 2-10 seconds worth of blow time before the compressor has to refill the tank.
I looked into it for the WK, but I didn't want to customize it or remove my spare tire, so I went with the easier and cheaper option of the Chrome Dual-Tone Motorcycle Air Horn that he sells on hornblasters.com. It's nice and gets the point across, but you won't be making anybody need a change of underwear.
EDIT: I'm not positive, but I think this was the one I had (it looks the same) and it was more than you'd ever need/want: http://cgi.ebay.com/ebaymotors/AIR-H...Q5fAccessories
EDIT AGAIN: Those guys are local to Tampa, so if you wanted to hear a couple of them before purchasing, you could go and check it out.