My 2p worth 
I have 2 of the A.C.Ryan PlayonHD minis so can testify they are very good players , silent and evolving with firmware updates but not yet perfect.
The YAMJ isnt quite what I would call a finished product yet with it lacking true skinning support but they say they will add that in time. Not yet tried it over my whole NAS folder (1700 MKV's) but have tested it over some lesser folders and it works well if a little slow at times. I tend to use the other app they support called ThumbGen with a customised layout (details on ACRyan forums).
As for network use .. I only have MKV's in SD but these play flawlessly over both a wired connection and using the wireless adaptor they sell.
The Sumvision Cyclone MKV v2 is very similar for playback as it uses the same chipset but it lacks firmware upgrades and has no YAMJ support.
Having used a WDTV (gen1) the Sumvision and A.C.Ryan mini I can safely say that the A.C.Ryan wins hands down with full DVD Menu support and much better subtitle support within MKV with YAMJ thrown in too.
All IMHO of course
Cozzy

I have 2 of the A.C.Ryan PlayonHD minis so can testify they are very good players , silent and evolving with firmware updates but not yet perfect.
The YAMJ isnt quite what I would call a finished product yet with it lacking true skinning support but they say they will add that in time. Not yet tried it over my whole NAS folder (1700 MKV's) but have tested it over some lesser folders and it works well if a little slow at times. I tend to use the other app they support called ThumbGen with a customised layout (details on ACRyan forums).
As for network use .. I only have MKV's in SD but these play flawlessly over both a wired connection and using the wireless adaptor they sell.
The Sumvision Cyclone MKV v2 is very similar for playback as it uses the same chipset but it lacks firmware upgrades and has no YAMJ support.
Having used a WDTV (gen1) the Sumvision and A.C.Ryan mini I can safely say that the A.C.Ryan wins hands down with full DVD Menu support and much better subtitle support within MKV with YAMJ thrown in too.
All IMHO of course

Cozzy