This works great as a Plex server. I am upgrading from an older Mac Mini with an external hard drive. I was upgrading because the RAID enclosure was failing and got to the point where I would have to restart the enclosure after every episode I watched. I was looking for something that had the ability to transcode, even though my files are all small because I spend the extra time to compress them, and I wanted something for other files as well. The old server was taking about 30 seconds to load a 30 minute TV Show, this NAS is only a few seconds. I have only had it for a few hours, but I am already happier then I thought I would have been. Comparing to the Synology NAS, the WD PR2100 is not as easy to configure and manage, but for a home office/small office NAS it is not bad, but if you are looking for a medium size business or larger, you are going to want the consider looking at the Synology NAS instead.
The only 2 downsides was the USB port is not usable to transfer the data from the external onto the internal drive, and I was not able to access the library files to move the library files over to keep the watched files marked as watched, but neither of these were a deal breaker.