Not the worst movie I've ever seen, but pretty damn close.
The good: this movie is one giant homage to alien films. Like Alien. And E.T. And nerd culture in general. The actors are stellar, and a lot of the jokes land.
The bad: the plot. It's a jumbled mess. The story takes second place to the jokes - the jokes happen, and they completely break verisimilitude. Secret agents chasing our heroes? Well, those secret agents are nincompoops. An alien breaking out of a secret facility, on the run? Well, he'll just be very risky and appear right before as many people as possible.
The ugly: there are just so many pointless scenes in this film. Like the beginning. It just happens and virtually has no pay-off. Not counting the last 15 minutes which also make no sense.
You see where Pegg and Frost improved over the years to get to films like Shaun of the Dead, Hot Fuzz, World's End. But this movie is just so...inferior, bland, and boring. Watch one of those other films instead - they are smarter, funnier, and feature virtually the same actors under the direction of a genius, with some next level editing to top it all off.