I believe the Alien series breaks down as follows: One masterpiece. One very good movie. One nearly unwatchable mess. One strange, odd, incorrect effort. Each of the films are really different, and if the batting average of the series had been better I think we would have such a staggering range of intriguingly dissimilar great films. Instead we have a range of movies that are bad in dissimilar ways.
Alien Resurrection is just a completely strange movie, a film that feels absolutely outside of the Alien universe. Jean-Pierre Jeunet is an awkward fit, and he never manages to really mesh his style with the elements of an Alien movie. But like Alien 3, Resurrection has certain great moments - this time more of them, and the scenes in between the great moments aren't so hard to get through.
It's also the first sequel to feel like progress has been made; the final approach to Earth brings the series where it should never go and where fans had been clamoring for it to go for decades. And while the movie has elements that are dated, perhaps the most dated thing is the fact that Winona Ryder is the co-lead. Imagine that today.