Not too bad, but not great either... It is actionpacked and the effects are great. As for the storyline, it could be loads better. There are the usual plot holes; how did the alien grow so big, so fast?What did it eat? And why are all the Alien movies an introduction to bad human decisions? I mean could it not be possible to make just one of these movies not based and fueled on human errors?
It's like, hey, we are on a mission, and we do know about the alien-threat, but let's go check out a weird signal, and you know what? Let's just send a small team in the middle of a storm to an unknown environment, and when someone gets infected, let us just skip all procedures and what we learned in training and allow them back on the ship. And while we're at it, why not trust the weird android, who has been on the freakin' planet for ten years with the aliens, and who is acting really suspicious AND has a lot of weird stuff in his room. And why not follow the android into a creepy chamber with eggs AND stick your face into one of them?? What could go wrong?!? (Btw, where did David get those eggs? And what was the greyish alien, he tried to friend? Where did come from? When and why did he sacrifice Elizabeth Shaw? Soooo many questions?)
As for the people on the mainship responsible for 2000 colonists, bah let's overwrite the main-computer and go through a major storm to pick up the remaining crew... knowing that something og someone killed the others on ground; infections or aliens, cause that's military procedure?? How stupid are they? And were there really no way of checking if the android, they brought back really was Walther? I know, I would be extremely suspicious, them looking so alike.
Finally, I did not get the motive? So the android is disgruntled, and don't want to be a slave to humans? Weak!
The new Ripley started out really annoying, but she was the only one, who didn't think the new mission was a good idea, so that earns her a star in my book. Also she did not scream as much as Ripley, and that was a plus too.
So all in all, not bad, the effects were amazing, but the storyline could be much, much better!