90% percent of a spoiler's function is cosmetic -- they look cool. But you're really only going to notice a significant difference at triple-digit speeds, and even then it's not a pronounced sensation, it's just the absence of some of the skittishness the cars get up above 90 or so.
If you're having temperature problems, a ducktail might help slightly, as the altered airflow might produce better intake through the grill. With regard to matching front and rear spoilers, I think it's valid, but again, only when you're trying to keep the car under control in curves where you're up above 90.
Other than that, it's for the look. And I'm not criticising by saying that. I have three different decklid/spoiler combinations that I switch between. I tell myself it's for the once-a-month track days, but the fun of the electric 964 wing going up is most apparent when some 12-year-old kid is giving you a huge thumbs up as you streak past his family's minivan on the freeway.
My .02
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Jack Olsen
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[This message has been edited by JackOlsen (edited 07-31-2001).]