I just got a WD TV Live box, and overall I think it is a good option for what you're looking for.
That said, I've barely gotten it hooked up and tested at this point. I haven't yet upgraded the firmware (which is an older version as shipped) since I want to do some reading up on it to be sure others find the latest stable. I've also barely tested its functionality.
It plays ts video files, but I can't remember if it has audio/video sync issues when fast forwarded or if I even tested it. I played .m4v files on it by just changing the extension to .mp4. Those files played fine, but did have audio/video sync problems after fast forwarding.
I'll try to get some more testing soon, but not until at least this weekend as I've got a string of shows coming up.