you may need the mils in the future but most likely not right away. after the cats have some miles on them and are not working as well as when new the rear O2 sensors will start throwing codes (ie check engine light), so to keep that from happening you put the mils in. (one set, on the rear O2's only).
quality and fit are excellent. it is made by pypes exhaust and i read a post by them in the OT area that the price is going up after the SEMA show where they are making a showing. So get them now while they are cheap.
1. i want to keep my low-end torque, cuz i dont drag race
2. lots of cops around here
3. emmissions reasons
4. i dont want it too loud cuz its a daily driver