Just had my flowmaster dual exhaust installed. 40 series muffler, single in, dual out straight out the back with dual stainless 3.5 inch tips. SOUNDS GREAT! No noticable increase in hp or fuel mileage the I really was only concerned with the look and sound. Nice deep rumble that seems to be the loudest between 1500 and 2000 rpm's. When you really mash it, it has a more throaty sound but is not as loud as I expected. I was told that this was pretty much the loudest muffler available. I will try to post some pics tom. (Friday)

 

also dont forget once the muffler gets broken in it gets louder. my bro has flows on his truck (not F-150) and its gotten almost 2x louder since he got the muffler put on. he has the single muffler with dual pipes out the sides. i dunno if different models make a difference in how much they change after breaking them in, but his changed a lot

 

When i had mine installed, my gas milage got worse for about 2 weeks...
Now its gone back up...love the sound.