I'm using 6 of the 2 TB Western Digital Caviar GP drives in my new build, in a RAID-Z array. Despite reading horror stories online, eg the many users seeing drives die quickly in the customer reviews at Newegg, mine are working great despite the sizable stress tests I've been running.
Except: one of my drives keeps reallocating sectors. I see this in its SMART diagnostics (5 sectors as of 2 weeks ago, 14 reallocated as of yesterday). This isn't normal (eg, the other 5 drives have 0 reallocated sectors), so I'll be keeping an eye on it and at some point might ask WD for a warranty replacement. I wonder if there's an accepted "policy" on how many reallocated sectors is too many? This reminds of the numerous "how many dead pixels are too many" discussions for new LCD monitors.
Of course, I don't lose any data because of this; ZFS's RAIDZ simply corrects the error for me.
I bought 3 of the drives from Newegg and 3 from Amazon. If I were more patient I would have spread them out over time as well. In general you should buy your drives across space and time, to minimize the chance of "correlated failures". If you buy all your drives from the same place, it's likely they were manufactured in the same "batch" which means any manufacturing defect in the production of that batch would make it more likely that you'd lose 2 or more drives at once, thus destroying all your data.
Newegg, it turns out, does a poor job shipping hard drives. They simply wrap them in bubble wrap and tape it up, sometimes packing 2 drives together inside the bubble wrap. What they don't realize is, because of rough handling from UPS, those bubbles pop, one by one I imagine, during transit, such that by the time I receive it, there is zero protection (no bubbles left) along at least one edge of the hard drives. It's rather shocking because Newegg is otherwise excellent. I've read several posts in the user comments noting exactly what I just said, yet Newegg hasn't improved. It's a bad sign when a company stops listening to its customers.
In contrast, Amazon (whose price matched Newegg's) packed each drive into it's own dedicated foam packing and box. Fabulous!
[EDIT Jul 28, 2009: I just received one more drive from Amazon, and they unfortunately have taken a turn for the worse! They now ship in a similar fashion to Newegg, wrapping the drive in minimal bubble wrap which pops during transit. They also take the wasteful step of "box within a box", which I don't think adds much protection to the drive. This drive will be my "hot spare", so if/once it get swapped into the array, I'll try to remember to watch for reallocated sectors and any other problems. Sigh.]
The one drive I see failing was in fact one from Newegg (I kept track of the serial numbers); it's entirely possible Newegg's poor shipping and the rough handling from UPS led to this drive's failure.
Losing one drive in a RAID array is quite terrifying because until you get the new drive resilvered, you're running with no safety margin! If you lose another drive, you've lost all your data. RAIDZ is not a replacement for good backups. It's best to have a spare drive on hand; you can even install it and notify ZFS to keep the drive as a hot spare, meaning if any drive drops out of the array, ZFS will immediately start the resilvering process to bring the new drive in. Or you could create a RAIDZ2 array, which has two drives worth of redundancy, but then you've "lost" 2 drives worth of storage!