i was about to buy the LG 42LK450 but instead decided on getting the LED version which is the 42LV3500.
i was able to test both the samsung 550 and the LG LK450 and both of them can play .mkv files.
what really made me decide on getting LG was (as aceaceace mentioned) is the easy warranty claims and the home theater add-on.
so far i'm enjoying my LG TV and HT without any problems...