Yes, it's for personal use...for recording HDTV, mostly. The average hour-long show runs about 7GB, and I tend to watch shows in batches. For example, I haven't watched any episodes of this season's "24" yet, and that's taking up 155GB of hard drive space. (That's kind of an exception...I love to watch an entire season of "24" in one weekend, when I find the time.) Other shows I tend to watch in 4-hour sessions or so. And with the way the networks are cancelling shows in recent years after just a short run, I don't even bother to watch most of them at all unless they've aired the majority of a season. That way when they cancel it, I just delete it. Anyway, HD recording eats a ton of drive space, and I'm tired of missing recordings because my hard drives are full.
Also, I'm a big fan of quiet, especially for my HTPC. (The Antec P182 I picked up from Fry's for $50 AR recently will help in that regard.) That's why I'm planning to move to diskless computers for both my gaming and home theater computers, with the only drives in the house being in the file server that's located in the closet in the garage where I can't hear it.
When I realized I could put together 4TB of space for about $1000, it seemed like a must-have. Sure, next year I'll be crying because the 1.5TB drives are on sale for $199 and I won't have used even half of the drive space that I bought this year...but it'll be worth it to have essentially infinte drive space, even just for a little while.
By the way, I just finished speed testing an array of five 7200.9 drives on my file server: 175MB/sec read, 70MB/sec write. I think I can live with that RAID-5 performance hit.