ECC protects against random "flips" of data in memory, like those caused by solar flares. Performance is only slightly worse...the extra parity bit is not read by the system, but it is used in a quick "check" that takes a beat every time the memory is read. This might bump up the latency by one cycle.
For some applications, these rare and minor errors could result in a disaster. If your computer was being used for, say, air traffic control or monitoring the electrical grid, a crash or incorrect calculation could mean Really Bad Things. A failure in a server used by a large company (banks, the IRS) could result in a huge business disruption.
But for most individuals, ECC doesn't matter. A single bit flip that occurs while you're playing Warcraft will probably not affect things at all.