The first film, stand alone, is decent anough. I enjoyed it enough at the cinema and kept me hooked til the end. The action was decent, the acting was...good enough and i wanted to see the brawls between aliens and predators. The second film however is definitely not going to go down in the history books. Its one of those films that you dont mind watching if you are lay in bed and it pops up on tele at like 12AM. Its got enough action in it and the creatures are well put together, but the story is poor. So if you want to see a film that is just purely explosion, close-up of alien, punching and biting lots then its ok.
Still, watch the first one though if its not too expensive!