I don't know how you could purchase a ticket to Rambo and not know what you are getting in to. I mean, it's Rambo. If you're expecting some sort of actual narrative or decent dialogue you're barking up the wrong tree. But if you want to watch John frickin' Rambo do his thing, with plenty of killing and gunplay in the mix... well, you're going to be a happy camper here.
Burma is the locale for the fourth edition of the Rambo saga. For the sake of simplicity let's call the Burma bad guys the "bad guys" and the Burma good guys the "Karen Rebels." The Karen Rebels are actually a real deal rebel group. You can Google them and read all about them; so Sly isn't just making this stuff up as he goes along. However, the main point of placing Rambo in Burma is to give him that handy jungle locale and plenty of baddies to "interact" with. It all kicks off when missionaries enlist Rambo's help in going upriver to deliver medicine and bibles to the rebels. But something goes wrong. DEAD WRONG.
Sorry, that just felt right. Anyway, Rambo faces off against a few hundred bad dudes - I'm not going to ruin that for you. Sly looks older, yeah, but still super juiced up and in better shape than 99.9% of the humans in the world. What I'm saying here is that he pulls it off. He has a giant head and he doesn't talk much but he's still completely and utterly badass.
If this movie has problems they are based on the genre alone. Americans (and the world) are tending towards more complex stories and Rambo and his outlook are anything but sophisticated. You cheer for Rambo to slaughter the bad guys en masse because they are pure evil. This is a purely visceral experience and it's only in cold retrospect that you'll question what it is about a headshot that pleases you so.
Perhaps it was nostalgia, perhaps it was low expectations - but whatever the case I found Rambo entertaining. If you didn't like the first three, or if you hate action flicks, or if you think Sly is too old for this sort of stuff than you should skip it. Everyone else... lock n' load.