I feel sort of "If that's all the effort you're going to put into the film, I'm not going to put any effort into reviewing it."
On paper, it sounds good, and on trailer, it even looks pretty good. Daniel Craig wakes in a Western desert, with no memory of who he is and how he got there, with a weird alien looking wristband, and a stinging arsehole*
He heads into town, only to find that the identity that he has quite forgotten is that of a notorious outlaw, and he soon finds himself in trouble, and under arrest. He's to be transported to trial somewhere, when all hell breaks loose, as lights appear in the sky, and UFOs start blowing the shit out of the place and kidnapping people. The wristband turns into a gun, and Daniel manages to shoot one of the (surprisingly small) UFOs down; the rest of the UFOs scarper with their captives.
The townsfolk find that since the downed UFO's pilot is bleeding and escaped on foot, they can track it, they put together a posse to go after them, and with a wince each time the horse jolts his poor punished ass, Daniel Craig rides along with them.
And they head off into the wilderness, run out of script and enter the land of Who Gives A Fuck. Really, everything I just said above was pretty cool and awesome, and found its way into the trailer, and everything else afterwards is just a bit of a drag. There are characters here, most notably Harrison Ford as a grizzled old ex-military guy, and it's not them that's at fault. It's that they have basically fuck all to do. There is a reason for this.
The aliens are fast moving, and live off screen, only appearing for brief moments of RAH JUMP IN YOUR FACE! They aren't interested in anything other than gold and Daniel Craig's arse, and have no dialogue, either among themselves or with the humans. In short, they are practically absentee bad guys. They're not even interesting in the Alien From Alien and Aliens sense, of being a bit mysterious and their lifecycle and habits being part of the puzzle of what's going on; they're just a bunch of uncommunicative gold miners and bottom-troublers. They are a blank area where the plot ought to have been.
As such, the thing just sort of ends with a badly shot, badly thought out, mess of a gunfight, which in principle has something to do with everybody working together in some kind of plan, but that's so because they *said* it was so, rather than there being any visible evidence of that happening.
Then the film ends and you can go and do something else. I quite enjoyed that bit.
Overall, I would say the problem with this film is that if you're going to make a film with a crazy premise, you've got to keep piling on the crazy, and really explore the outrageous thing you came up with, not just say "Well, we have cowboys and we have aliens. Job done, let's have lunch."
* There is no direct evidence in this film that Daniel Craig's character has been anally probed, but it's an entertaining thought, and one of the few suggested by this film.