A mate of mine had one, and claimed it was so loud he could set off house alarms let alone car alarms.
So I had a go (after calling him a few names along the lines of 'liar'), and counted 36 cars & 3 houses. Man that thing was loud.
When stopped, I remember a copper saying "That's a rediculously loud exhaust isn't it?" To which my mate replied "Yes, yes it is!
"
Get one, you won't regret, even if it's just for the noise.