I have a DS1812+ with 4 x Seagate 3 TB drives in SHR - around 8.8 TB in total.
I just installed the DS a few days ago, so I haven't set up everything yet - but its key roles so far are:
1. TimeMachine target disk for 3 Macs (2.5 TB allocated for TM)
2. File server over AFP (for the same Macs)
3. Media respository for Plex
#3 is really the primary reason for buying this NAS, and as I rip a lot of Blu-rays I need a lot
of space. A Blu-ray ripped using MakeMKV (i.e. no compression) typically consumes around 20-30GB of disk space for a movie, or 40-50 GB for a disc containing 4-6 TV episodes. It all adds up pretty quickly, and I have around 3 TB of video in my library - a mix of M4V and MKV.
I also needed a fast NAS, as this video is potentially streamed to a bunch of sources simultaneously - and I still need the NAS to be responsive as a file server while all this is going on, so that I can copy files from/to it without interrupting the video streams. The 1812+ served my needs perfectly in this regard - this thing is fast
In regards to the previous poster, I don't run the Plex server add-in on this NAS - instead I run Plex on a Mac mini with volumes mapped to the NAS. This is mainly for performance reasons, as streaming and transcodes can consume a lot of CPU and RAM, and even though the 1812+ is fast it can't really compete with a bonafide Mac that has a faster CPU and 8 GB of RAM.
TBH I've found DS1812 + Mac mini + Plex to be the perfect combination for a home media server setup. It's costly, but imo worth every penny