We use consumer-grade drives across our entire 40+ petabyte storage farm because we do not need high-performance drives. Instead, we need low-cost, low-power, high-density drives to put into our custom-designed Storage Pods: http://blog.backblaze.com/2011/07/20/petabytes-on-a-budget-v...
We tested "enterprise drives" and found that from a pure reliability perspective, they actually performed worse in our testing than the consumer drives. With about 5 years and 40+ petabytes of data, we're fairly confident this holds true.
Finally, we assume that all drives will fail and replace about 10 drives every week. The software is written such that the system takes drive failures into account and data is stored redundantly and continuously cross-checked.