I've never bought one and never needed one. If you do your research up front you know you're buying a quality product that should last well beyond any warranty period. The one exception was my 1999 Jeep Grand Cherokee, which was a piece of junk, but fortunately I leased it (I never lease) and dumped it at the end of the lease even though they begged me to buy it. I'll never buy another Chrysler product.
It might be wise to get an extended warranty on a car stereo. At Best Buy, the extended warranty includes installation issues.
They are worth it on gaming systems. Sony has no warranty on the PS2, and ours broke twice. I also got one on my iPod.