Even though SSD HDDs are starting to gain ground as fast and silent boot/system drives, they still can't compete with traditional spinning magnetic HDDs for capacity, or price per GB.
The latest HDD from Western Digital is the Caviar Green 3TB with a 64MB cache. Initially this is costing £162 plus VAT, which is a lot when you consider a 1.5TB drive can now be bought for £46, so 2 of those totalling 3TB would be roughly half the price. And even worse, a Samsung 2TB drive can now be purchased with VAT for just £56, so you could get 4TB for just £112. Or twice the storage uing 3 TB drives for the same price; 6TB for £168. But for mass storage, a single drive is always nicer then 2 drives. And priced will definitely start to drop once they have been on the market for a while.
In review speed tests it seems pretty fast for a slow Green drive too, returning large file test results of 110MB/s and 169MB/s, which isn't as good as the Samsung F4 tests, which returned 138MB/s and 208MB/s. But it is still pretty fast, and more than enough for a media drive or general storage usage.
There is one downfall of a 3TB drive, and that is hardware and OS compatibility. Most existing NAS and media systems only support HDDs up to 2TB. And Another is that Windows can only boot from a disk larger than 2.19TB if you have a UEFI motherboard and a 64-bit OS. Such motherboards are very rare at the moment so these 3TB drives will mostly only be for secondary storage, and even then, a lot of motherboard SATA ports won't work with the drives and instead a special bundled PCI-E to SATA adapter will need to be used instead. In addition Windows XP doesn't support drives of this size at all, so only Vista/Win7 users can use them, plus recent Linux Distros. Also there are reports of compatibility issues with all 32bit OSs, so even 32bit versions of Vista and Win 7 could be effected.