I'd love to be able to publish these statistics for our organization, (I'd estimate we have close to a quarter million drives in the field) but there is a big hurdle in the way: legal liability. If I was to say something negative about Western-Sea-Tachi drives, their lawyers might call our lawyers, and we could easily spend a million in court fees.
The thing I think would be interesting is that we have a completely arbitrary mix of drives, based on drive availability over the last 6 years or so. We also have a mix of different service companies who replace the drives in our workstations. Our contract is such that we don't control the brands, or even the sizes, as long as they meet or exceed our specs. As a service organization, they're responsible for picking the cheapest option for themselves. If our spec says "40 GB minimum", and they can't get anything smaller than 500GB, they'll buy those. If 1TB drives are cheaper than 500GB drives, they'll buy those. And if we're paying them $X/machine/year for service, they can do the reliability decisions on their own, so if they think some premium drives will last two years longer than stock drives, they might be able to avoid an extra service call on each machine if they spend $Y extra per drive. I expect these service organizations all have their preferred drives, but that's not data they're likely to share with their competitors on the service-contract circuit.