I have used both the WD TV HD media player and the WD TV Live for almost a year.
I don't find the UI "clunky" at all. What I find clunky is the Windows 3.11 File Manager UI of most other media players like the Asus O!Play. While the UI of the WD TV Live is not "eye candy" it IS very functional and easy to use. If you have young children or grand children, they can use the WD TV Live with just a little bit of teaching since the UI can be thumbnail based.
While there are other players that cost less, the WD TV Live, for $120 U.S., is a good all around device and an affordable addition to a Television setup. It also has a small footprint for those of us that are space challenged.
I, personally, do not have any issues with the subtitles on the unit, especially after the latest firmware upgrade that added PGS support and the ability to reposition internal and external srt subs with mkv files. I have not tried it with external subs with avi files, but I imagine it will reposition them there as well.
Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways,
totally worn out, shouting '...holy sh*t ....what a ride!'