Much of the ECC versus Non-ECC argument comes down to speed versus reliability. The reliability argument at least is easy to validate; over the last 12 months we've seen a tiny .09% failure rate with ECC memory compared to a .6% failure rate for standard memory. As for performance, we've actually tested RAM performance in the past and found that faster memory does not always equate to better performance since the RAM is often not the bottleneck.
However, the expected performance drop of ECC RAM is slightly different than what we saw in those tests since it's due to the extra error checking process rather than slower timings or frequency. In fact, Crucial has a statement on their knowledge base that ECC memory will decrease your computer's performance by about 2% compared to standard memory.
Alas, to settle the question of whether ECC actually lowers system performance we ran a series of benchmarks with standard RAM, ECC RAM, and Registered ECC RAM that all run at the exact same frequency, timings, and voltage.