'vplayer' works rock solid for me (no pun intended!)... the alpha one (yeah, I ante'd up and paid the donation, well the first 100 user donation that is...even though the devel had said it would stay free forever, I love the app and felt he earned compensation). I use it on the Tab (Sprint) i have gratis (thanks to work).
I played a mkv of old school 720p and and a hi-def wmv (pr0n, lol) ..both work fine w/ 0 lag. All my xvids and flv's and mp4's have worked flawlessly. I ripped the old school file myself from a hd-dvd copy I had used handbrake on, a longgg time ago.
Ill try some other mkv's that are newer...Girl With a Dragon Tattoo...and a few others when I have some time away from work.
I'm thoroughly impressed...of course, I was stuck back on a TP2/WM 6.5 for a handset, so of course I stand to be impressed, lol. I was using my lappy for any mkv / 'hi-def' content prior.