Crazy. I hadn't even seen any 6 TB drives for sale, yet. I figured those were the next big thing and still a year or so away from being reliable (word of advice, never be a hard drive capacity early adopter, I've been burned).
Anyway, at this point in time, I find 3-4 TB drives to be the sweet spot. I have no idea what my overall data storage is right now minus back-ups, as I have a ton of hard drives, but if I was going to pick up another drive today I'd probably go for 4 TB and be comfortable for a good while. 8 TB is likely to be prohibitively expensive for the speed at which I'd actually use it up. In a couple years, or if 4K video content starts to really take off in the consumer space before then, I could see 8 TB being the standard for my use cases, but I'm just not there yet.
If I was in the market for another 8 TB of storage, though, I'd probably go for two 4 TBs, instead, and either RAID them or just have two disks. Probably would be cheaper and certainly I'd feel safer with them. I'm not working with multi-terabyte files (yet), that actually require an 8 TB single volume.