Had Magnex, look nice, sound dull and dreadful on a long motorway journey and wouldn't fit quite right. Tried a different Magnex after that and it was quieter and okay on motorways, but sounded dull and boring.
Had a scorpion one, didn't last long before I got rid of it, the backbox hung low and it looked crap. Sounded okay, not too loud, build quality was hard to tell, but it had a LOT of surface rust.
Then got a Powerflow, one of Top Gear Motorsport's ones (based in Burnley) and that was VERY VERY loud, but sounded like a touring car at full chat and had a nice non chavvy rasp (the F7's get away with it imo, the F4's sound shite).
Sold the car, but that system was too loud for the owner, so got a Magnex to replace it, which was actually pretty quiet.
That was just on the Williams, the other system I had on it was half Magnex and powerflow, so it's only 7 systems thinking about it, as I had a straight through center section that I used on the powerflow.
172 had a Powerflow on it when I got the car and it was awful. It was only 2" bore, was quiet, but fell to bits and the back pressure blew an 11" hold in the backbox!!! Replaced that one with a GDI system and am 100% happy with it. It's a bit louder than the normal range as I wanted a 3.5" slash cut thin jap style system as I'm sick of inward and outward rolled ones and it's barely visable, but I pay for it in noise levels, but it's easy to live with daily still. It was much quieter on ITB's when it was built on the car though.