To everyone except those with a trained eye, they're virtually indistinguishable; however, because CZ isn't as hard, they tend to scratch and dull over time and thus lose their luster.

If we're talking about 'quality', CZ just doesn't compare for longevity. That said, unless you're talking about a daily-wearing-for-many-years piece of jewelry (i.e., an engagement ring), CZ is just fine for most folks, especially considering the cost difference.