Which is a shame, as it's not bad at all.
Planet 51 is effectively Planet 1951, being very like America in the 1950s. At least, like one if everyone has greenish skin, antennae and no nose, and the wheel has been displaced by levitation pads to such an extent that even a VW van hovers. But it has comic shops and the first sight of hippy protesters, and a xenophobia powered by films of alien invasion. Into this lands an astronaut, Chuck Baker, who's supposed to make a few steps out onto an uninhabited planet, plant Old Glory, and return to his command module, there to press the button that will fly him home again.
Sadly for him, the planet is far from uninhabited, and when he stumbles through a barbecue and gets separated from his craft, the anti-alien hysteria latches onto him, and he spends the rest of the film in jeopardy of getting either just left behind as the orbiter heads home automatically, or cut up for experiments in the infamous (and, of course, totally non-existent) Base 9. Happily for him, he has the help of a local, Lem, who is just starting his job as assistant at the local planetarium.
It's a delightful story, one that explicitly hangs a lampshade over the fact that both humans and aliens are speaking the same language, and implicitly hangs another one regarding the 1951-edness of the planet. There are a lot of nods to other SF films, from the Close Encounters cloud maelstrom to the E.T. bicycle in front of the moon, as well as other references (surely Lem's name is one). But there are also a number of cute characterisations, from the alien (and very Giger-styled) 'dog' that urinates acid to break its chain, to the astronaut's small 6-wheeled robot probe which, being called Rover', also behaves like an intelligent dog.
There were a number of places where we were laughing out loud, and I think we'll be buying this on disc when it comes out. In the meantime, this is recommended, if you can actually find it on a screen.