its louder for sure, and it sounds more like 12 oz pop cans cracking open at high frequency.. if you have headers its still quiet but even more tin-ey as ViciousJay stated. but I've heard they are much quieter than the gt500's..
I think they are more restrictive also, but i'm not sure.. still investigating.
MIke with the white Mustang has a video comparing stock mufflers to GT500's.. the stockers sound great in the car, its out of the car they have issues.
Stock mufflers with no resonators sound like crap outside of the car. Inside, they are nice and quiet (if that is your thing), outside they are like canned ass when you open it up.
If you want to add some sound, but not an excessive amount, and have it still sound nice, GT500s really are a great choice and inexpensive.
I just ask Jay this question, but what do you about running an ORH + GT500 with the resonators still installed? I can't really find any videos on YouTube that state this setup.