HDTVs look great with HD sources, but when playing analog signals, they look even worse than the analog TVs they replace. That's one of the dirty secrets of the HD upgrade; 480i games and broadcasts look weak. If gaming on a TV with component inputs, those cables are a must; they even make Xbox, PS2, and GameCube titles clearer. (And for fully HD consoles, cables that carry digital signals -- like HDMI -- are best.)
Wii buyers with 480p-or-better TVs, be sure to budget those cables into your console purchase.
[Update 1: PS3 users with HDTVs, you're going to have the same issue. Somehow we forgot that the component or HDMI cables cost extra. Thanks to all the readers who pointed our the error.]
Read: Hil's Blargh! - Composite Cables - The Wii Killer
Read: Wii Component Cables!