I have seen this film, and enjoyed it a lot.
As a sci-fi-horror-comedy it's not overly thought-provoking, not the scariest, and not the most laugh-out-loud film I've encountered. However, it's a fun and entertaining flick, which oozes charm, has some great dialogue, and the direction had snappy set pieces and "jumps" in the right places.
The largely unknown cast members were excellent and well supported by Nick Frost doing what comes naturally. The performances sold the film for me - the actors felt at ease in their roles, the performances slowly grew on me, and I eventually felt compelled to root for the likeable central characters. The actor playing Moses in particular was very charismatic and helped carry the film. I hope they all go on to greater and bigger things.
The film kept me engaged, amused, thrilled and shocked in the right combination. I loved it, and would recommend it to anyone looking to have a fun time at the cinema. A great first feature for Joe Cornish, and here's hoping he makes more great movies.