I found this film barely watchable, I hardly liked anything in it and I forgot most of it hours after watching it. Below, more of my impressions with some LIMITED SPOILERS.
In 1873, Arizona Territory, an unnamed loner (Daniel Craig) wakes up in the desert injured, with no memory and with a strange metal object on his wrist. I will say no more about the story, obviously to avoid spoilers but ALSO because there is really not so much to be said... Later he will have some adventures, during which he will meet a preacher (Clancy Brown), a mysterious woman called Ella (Olivia Wilde), a saloon owner (Sam Rockwell), an aged, powerful local cattle baron (Harrison Ford) and later also a rough and tough and yet slightly clueless bandit named Hunt (Walton Goggins), who may or may not be somebody from his past... Some Indians at one moment will be involved. And yes, as the title clearly indicates and as the trailer suggests, there will be also some aliens. Adam Beach and Keith Carradine appear also in cameos.
Now, with a minimum effort from the scenario writers (led by none else than the infamous Damon Lindelof), director (Jon Favreau) and producer (Steven Spielberg, no less!) this could have been a honest action film or at least a good comedy of western/SF persuasion. But well, Spielberg probably never even had one single look at the scenario, Damon Lindelof (the guy who wrote this "Prometheus" abomination) did his usual thing and the director just went with the boring and ridiculous scenario without giving a darn about the final result. Which is awful!
This is a very boring film, with poor action scenes, bad dialogs, virtually no humour, ridiculous story line and a pathetic great finale. The only reason why I give it two stars, is because of presence of some good actors, amongst which one (Harrison Ford) I like a lot and another one (Olivia Wilde) is a hot chick. Other than that there is nothing there. Avoid.