Used to run straight-piped and got tired of it... it was just too loud, and there are those times that I like to sneak around (not that it's easy, but it's alot easier w/ muffler on there).
Also, I think Rip's 4" sounds better at WOT than a straight-pipe. When under load, and/or light acell it sounds just like a big-rig and gets you the looks you're looking for.
Am looking for more quiet in the cab though... will be doing sound-deadening and maybe TAG soon. Outside is another story... I don't mind if it sets off car-alarms (get 1 or 2 a week as is) but either way is good for me.
Josh