I have a Magnaflow cat-back on the 84 Camaro, only system that lasted more than 3 years on that car, anything short of stainless steel would dissolve in a couple of years. Sounds great and fit great from what I can recall.
My Sonoma has a Magnaflow hi-flow CAT, replaced a failed OE CAT and has been working great for years. Never an Emission test issue, and it seemed to wake the truck up a little bit.
Only negative experience was installing on a 350Z roadster build at work. Had to "clearance" one of the pipes that was interfering with something (i can't even remember what) under the car, and then centering the tips really sucked - some modification required, and still never got them 100%.