i have a regular flowmaster 50 series (not delta flow). I like it, its definately louder than stock at idle and sounds very agressive at 3/4 - full throttle.
My set up:
I have a borla header, k&n filter in the stock air box, new universal catco cat from summit, and i got the flowmaster 50 series and 2 1/2" pipes from rusty about 3 years ago.
Good results, i like it its a little loud on the freeway but i like it, if you are into that sound its a good healthy sounding rumble not junky sounding like a blow out muffler like some set ups with the wron g flowmaster. Also i keep seeing guys recomend 40 series. i do not purport to be a muffler expert but if you call flowmaster tech dept you will fine out for a stock / almost stock 4.0 (non stroker) you will find out the 50 series is the recomended muffler.
Also the 50 delta flow i have not heard in person it was not out when i got my orginal 50 series .... but flowmaster advertises it as canceling in cabin reverb and tones and still has aggressive sound for the street, its more or less using noise cancelling to lessen in cabin sound.