I officially have a 2TB HDD in my PS4. It was a super easy process, even removing the drive from the enclosure. The most difficult part was downloading again and reinstalling everything. I'm glad I upgraded now instead of waiting a couple years. I currently have 14 disc based games and about 10-12 digital games. My stock drive only had 40GB free. I don't think I will ever need more than 2TB. At the point when 2TB of games are installed, it may be time to remove a couple.
To anyone on the fence, I highly recommend doing this. Even if your worried about opening the system or enclosure up, it's not very hard. Sony has made upgrading the HDD really simple. And there are many guides and tutorials online that walk you right through it.
http://www.bestbuy.com/site/seagate-backup-plus-slim-2tb-external-usb-3-0-2-0-portable-hard-drive-red/5127041.p?id=1219111162308&skuid=5127041&sellerId=
That is the drive I bought. For $99, I upgraded my PS4 HDD and also gained a 500GB external for anything I want. Mine is going to be hooked up to my Xbox One.