We have been using WD RE enterprise drives in our NVR systems for 8 years. When the Purple models came out we started using them in our small (24 ~ 36 cameras) desktop 4 and 8 drive NVR's. The Purple drive performance is good for these smaller systems. We use the Purple NV drives in our 12 ~ 16 drive Performance NVR's (36 ~ 72 cameras) and WD GOLD in our 16 ~ 24 drive Enterprise NVR's (72 ~ UP cameras) . We do 48 hour burn in and performance tests on our systems before we deploy them. This weeds out any marginal drives so our field reliably of the purple drive has been excellent. I hope this helps.
This is somewhat backward looking, but of the infrastructure we're monitoring, 2TB drives dominate the list (37%), followed by 1TB drives at 24%. Drives larger that 4TBs represent around 1.5% of our population with most of that being 6TB drives.