i have had a powerflow done twice (once for my old N16 almera and one for my new civic), on both occasions i have to say i am very pleased, did exactly what i wanted in terms of sound and look and the welding was very well done.
Remus backbox is a fantastic back box though. With a straight through mid section and a remus backbox you get a beutiful sound (had this on my almera at one point). The sound is not loud at all and is basically stnadard until high revs. Personally i prefer the light sound and discreet look of the remus tailpipe aswell