Corsair CX series are rated as tier3 (except the CX400), that makes them average psu's that will do pretty much as they say. Average lifespan, average dependability, average DC output, average capacitors etc. Only 1 seasonic is tier3, the rest being tier1 and tier2, and by default, that makes most seasonic above average, and that includes the non-oem brands like XFX and Corsair AX/HX.
That said, my definition of a 'good' psu is one that works, does the job intended to, lasts as long as supposed to and makes no complaints. For most systems I have no worries about using a CX series psu, they are about as average as the rest of the system. When it comes to enthusiast class systems however, you get what you pay for. High amperage gpu's, oc'd chips, multiple fans, leds and other gizmo's, low latency memory and high precision mobo's and you want an average psu to match? not likely. Be like putting P-Diddy in a $2000 suit... with $2 flipflops