I have to admit that my FX is pretty much trouble free for a 9 year old car. I only had to change a sensor and brake pads. Soon, I will change my driver seat track due to a bad seat motor and broken seat bracket. The driver seat motor died because the automatic entry feature was on for about 6 years and wore the forward/reverse motor out. The seat bracket broke probably because I corner hard around a couple roads with twisties near my house for fun often. But compared to wife's 06 Murano, which I seem to repair almost twice a year lately, it is a tank. However, it does have minor build quality issues like rattles,dash, poor quality seats for a luxury car, faulty gas gauge, and the FX45 seems to need to be top off with a quart of oil before a listed 3750k old change due to a common oil consumption problem due to sticky piston rings or piston rings with tension too low to provide proper oil control. But, it still drives great and have not left me stranded. I would say I would buy another FX, probably the FX50S next.
I've just broken 217k on my 03 and my seat track has broken twice; the first time i opted to have it replaced at Infiniti to the tune of (if i recall correctly) $1500. At this point i didn't think it was worth it so i took the seat out to see what was going on. It was an easy weld at my buddy's shop and now it's good as new. Just something to think about and maybe saveyou a few bucks. Fortunately i haven't really seen any of the build quality issues you speak of. I too am thinking of moving to an fx50, but I'm kind of leaning towards something with off-road ability.