In fairness to Monsters, blame the studio. They marketed it (wrongly) as an invasion film when essentially its a road movie and a pretty good one at that.
The director, Gareth Edwards, hails from Nuneaton (just down the road from me) and what he achieved on the budget he did is pretty remarkable and as a result is a great inspiration to filmmakers like myself.
But yeah - its not an invasion film and if you went into it thinking it was, then its Vertigo Films / Magnet Releasing's fault, certainly not the directors. And let me be clear - your perception of what you expect from a film going into 100% affects your final judgement. No doubt about it. I went into watching Monsters expecting a road movie and I totally enjoyed it. Had I expected an invasion film, then yeah, I don't think I would have liked it as much.