I never actually saw Ong Bak so I can't comment on similarities. But there's much to enjoy in Merantau: the buns of steel, the dialogue of wood, the fights of brutal poetry, the hilariously timid subtitles, and the fact that Christine Hakim seems to have wandered in from an entirely different movie. Oh yes, the political subtext, with its parallels to Avatar (notice how the villain looks like Dubya). I wonder why the distributor didn't put in Malay subtitles on top of the English & Chinese ones though, especially for the Minang scenes.
The version screening here is the international cut, which is about 30 (yes, 30) minutes shorter than what the Indonesians saw. I don't think any of the fight scenes were dropped, though.
There will be some people who will recommend this film for 'introducing silat to the world.' I guess these are the same strange people who will watch Avatar because they care about rainforest conservation. A popcorn flick is a popcorn flick, regardless of whether it comes in a Padang flavour.