Good: OS X has built-in spellchecking in applications that choose to support it.

Bad: Thousands of common words are missing from the dictionary, especially place-names.

Worse: It completely ignores all words that are three characters or less, so forget about OS X helping you when you mistype 'teh' or 'iz' or say "ack! t his is soo wrong!"

Sometimes an ounce of cure generates a false sense of security that can end up doing more harm than good. When you don't see any more red squiggles and you assume everything's spelled right, it might just be that the OS isn't looking close enough.

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Addition: Are there spelling bees in Spanish-speaking countries? Spanish words are pronounced strictly by their spelling (or more likely, vice-versa). Is the spelling bee a happy by-product of a disjointed, inconstant language? It'd be a shame if other countries never got to experience the joys of spelling bee championships.