All.
I'm probably one of the QX56 owners that's less enthusiastic about owning one than some others, so take my recommendations with a grain of salt. We purchased the QX to tow an RV, and later got rid of the RV due to not having enough time to use it. We've been through the timing chain replacement (thanks MaxCare), and have had to replace both of the cats out of warranty (damn MaxCare). Since I've been responsible for paying for both of those cats, I was more interested in cost than longevity, since I'm not going to be keeping this truck after the warranty runs out (currently in 45k miles). ALL THAT BEING SAID... we found non-infiniti replacement Cats over at Rock Auto. They're made by a company called Davico, and according to my garage, they're a perfect direct factory replacement. They have a 50K warranty, and so far, have been doing what they're supposed to do. I have no idea how long that will continue to be the case, but if I can spend $400 on a part instead of $850, I'm ok with making that choice. YMMV, but I wanted to make everyone aware.
Good luck,
BigdogBTS