I'm using 10+ Hitachi 1TB and 2TB disks in raid 1 and raid 6 right now. They have been working flawlessly. Plug-and-go. Peak performance isn't as good as faster drives. Power consumption is a little higher (we are talking 1-2w v. other 7200rpm drives). The drives just work though. This year I've RMA'd 0 Seagate, 0 Hitachi, 0 Intel, 1 OCZ, and 3 WD (green 1.5TB). Also the greens were not happy on the Areca raid.
In fact, when I added my now third 4U case to "The Big WHS" yesterday, the first drive in the enclosure was a Hitachi 1TB drive.
I still haven't RMA'd a Hitachi drive yet, so on one hand I can't comment on their RMA process, on the other hand, I haven't had to use it.