ECC stands for Error Correction Code and does just what it sounds like it does, detects and fixes errors in memory data. If your motherboard supports the use of ECC memory it's probably a good idea to use it.
This is only needed if you have tons of ram in your system and especially needed if you do multitasking alot. ECC slows down performace of your ram by 2 to 5% Servers generally use this type of ram. For a gamer, stick with Non-ECC because its cheaper and better.
Just a quick note, you are correct on the slow down..as extra writing needed to make sure it can correct.
BUT ECC ram corrects 95% of all errors that ram will eventually get...It will correct 1 bit errors, which when ram fails (95% of the time) that is what makes it fail...
So you have the weigh the benifits...With the price of ram right now, benifit is low, but when ram was more expense the loss of speed by the ram fixing the error would save you the cost of buying new ram.
I fix computers for a living, and the majority of stability issues are bad ram, due to a 1 bit error...so if these people where using ECC they would of had a stable system, instead of constant lock ups, and would not of had to by new ram to fix there problem.