Interesting here. I get a little ticked when people complain about me - I'm running 1 12" sub with about 200 RMS power, IB style, with two pairs of coaxials off of head unit power atm. At its max volume, the whole setup, it's at about my limit, but I don't like listening to it lower than that. I keep it at that level.
But... I'm not running a hell of a lot here. However, I get complaints from others at times. Seriously, how loud is too loud? Like others said, I get drowned out easily by some car with a modded exhaust (or, in some cases, rusted and beaten up exhaust), or a bike. However, a cop was about to pull me over until I turned mine down, rather than go after some guy on a loud *** harley.
Pisses me off at times.
Edit: Mind you, I only listen with the windows down, except on the odd saturday as long as I'm not driving through the middle of town. And I turn it down at stoplights regardless.