Alloys look better than hubcaps. alloys also come in different sizes, and the theory is that more metal=less rubber and more metal=looks nicer. you get to see your rotors and calipers, nice stuff. alloys give you the option for higher performance tires, although they do cost more than ones for smaller diameter rims.

Alloys are lighter than steel rims and this means less unsprung weight. unsprung weight on the car is anything not cushioned by the suspension: eg. wheels, tires, brakes, etc. the Unsprung weight on a car compresses the shock absorbers and springs before the rest of the car feels it. less unsprung weight means less inertia and less work is needed to keep the tires planted on the road. being made of aluminum or other metals instead of steel makes the alloy wheels lighter than steel rims, and they provide better handling.

my volvo (850) came with 15" alloys. it was a pleasant suprise. the car looks pretty sweet, and they look way better than the old hubcaps on my 240DL. i'm switching off my winters now and i'm looking for another 15" tire. the most important things to me is a tire that generates a minimum amount of road noise, and provides a good responsive ride, not too harsh or too squishy. basically it's the same criteria as buying tires for rims.

i don't race, i don't slalom (much), but i love to drive. Mostly i look for a general use road tire. if you have a more specialized purpose in mind, i must sound pretty undemanding!