Some decent sci-fi ideas in this one, although the portrayal of the space elevator was a little off. (It'd have to be on the equator, and there aren't many cities there; also, it might be a little unsafe to have it right in the middle of a city. And traveling up one all the way to geosynchronous orbit would probably take days. And there'd have to be more cable stretching outward from geosynch orbit, because that's just the cable's center of mass.) Still, nice try to include one at all.
And it was irritating to see them talk about cheating a retina scan when the special effect and the dialogue indicated that the gadget was working on the iris (the colored portion around the pupil, just under the cornea) rather than the retina (the light-sensitive surface at the back of the eye). And the scanner we were shown was clearly an iris scanner (which can work from a distance) rather than a retinal scanner (which requires getting the camera right up against the eye so it can see through the pupil). This is an irritating thing that is far from unique to this show -- iris scanning is actually a simpler, more reliable, and more common form of biometrics than retinal scanning, but the term "retinal scan" had already gotten embedded in pop culture by the time iris scans started to be used, so people keep using the old term even though it's usually wrong. Still, you'd think anyone who took high-school health class would have learned the difference between an iris and a retina. Certainly former
Stargate writers should know what an
iris is.
That twist in the last act with the swirly black mass taking over Three was weird. They wrapped up the main story early and then just sort of tacked on this epilogue that didn't really go anywhere. I'm sure it's a setup for something later, but as a piece of this episode, it didn't work well. Although I love how clear the difference between possessed Three and real Three was. The real one thought of Two's welfare first and only then caught on that he was in the isolation room.
Re: Four and Nyx, I think this is the second time in two seasons that we've had a couple of shipmates have casual, dispassionate sex and then insist on it being a casual, meaningless thing and then get uncomfortable about it. I wish these writers would find a less cold and awkward way of dealing with the subject. Either let the characters actually develop a relationship, or let it be actually casual sex in a mature, friends-with-benefits way rather than having them be all weird and uncomfortable about it.