I have an older version of this, the WD TV Live which I have mixed feelings about.
My box will play just about any format, but getting to play anything can be a bit of a chore because there is some serious lag in the UI. My wife who is less tech inclined doesn’t like to use it if she can avoid it because it is soooo slow. From what I can see on the mothership reviews, this wasn’t fixed in this version and probably won’t be because they have had this issue since the original WD TV which I also owned.
Also, from reading the amazon reviews, this unit discarded the Netflix support (which in my experience is only so-so on the WD TV Live anyways) which may or may not be a dealbreaker since so many devices play Netflix these days, but something to keep in mind.
My suggestion would be that if you are mostly concerned with playing local/network content and are tech inclined (or at least willing to put up with a bad UI) this is the way to go. If you want something that’s simple to use and is focused on streaming from the cloud, go with Roku.
I have the earlier version of this myself, but I really like mine. The only lag I get is it doesnt always connect to network the first time and cycles up again. This is the only box I have seen that plays ISO files without having to run other software like plex or something to get the stream going. Mine has 2 usb 3.0 ports with 2.5 t of drives that are network accessible, so I can convert my DVD or BluRay content to ISO then upload to the local storage for easy viewing. I love being able to watch an ISO, stop in the middle or somewhere, then come back later and resume in the same place. My dvd/bluray doesnt do that, as far as I know. I like this device, I sometimes think about Roku or Chromecast, but this does exactly what I need it to do. Caveat- my wifes nexus 6 does not do Miracast, so I had to get a Chromecast just to cast her phone on the TV. Not everything supports Miracast.