I have three of these drives running 24 hours a day in a ReadyNas NV+ RAID unit, and I've been very impressed with their reliability. They have each been running without failure or interruption (other than occasional moves) for almost 25,000 hours (that's just short of three years), and the in-built health monitoring shows the drives in great shape with no recorded ATA errors. This is despite several power cuts which have shut down the drives instantly.
I have been very impressed with these drives, and plan to expand my network storage using them in the near future. Great stuff!
EDIT - Just under one month later.....
....and the most recent of my three 2TB WD Caviar Green drives began making my NAS unstable, with a SMART health multi-zone error rate of >110,000. I immediately regretted posting the review above - talk about tempting fate! To WD's credit, a replacement arrived two days later and is installed and working now. Their RMA process was pretty simple, and if you provide credit card details they'll forward you a new replacement and give you 30 days to return the broken drive, which is by far the best RMA service I've ever had to use. Still recommended!