The feel, and atmosphere of the film was really well done, the science gone mad, and futuristic industrial (brown, brown everywhere!!) look, amazing. The real effects were really good, the looks of the xenos, was great, very slimy. I enjoyed the time passing montage. The characters were pretty good as well.
As for the Xenos themselves, the intelligence, three jointed legs, gracefulness, ability to spit acid, and the sounds they made in RES were awesome.
Not the best film, but still very good.