As a general rule prequel movies aren't that bad. for me they are a bit like sequels. If they are done well then they are worth it.
I didn't like the AvP movies and I love the series. This was mostly becuase there was to much of what I call 'the Human Factor', a bit like TFTM 2007. Too much focus was placed on the humans and not what the movie was meant to be about.