Actually advertized as ALIEN sequel
When Intruder Within first came on TV (as a two-part mini-series, iirc), several magazines actually touted it as a sequel to Alien (or prequel, maybe, as it took place in the present). Starlog was one such publication, and I think even TV Guide called it a (pre)sequel.
My memories of long-gone 'zines are fuzzy, but I vaguely recall an interview with Ridley Scott (or someone close to Alien) who admitted that it was, yes, a sort-of unofficial sequel. (I got the impression that even as early as 1981 they were tired of chasing down all the Alien rip-offs and just "went with the flow.")
I found Intruder Within to be rather slow-moving, but I'd probably get it on DVD (mainly for the Alien connection). There was one amusing speech by a paleontologist who was puzzled by the fossil record in the part of Antarctica they were exploring: Antarctica teamed with life until three million years ago (the age of the alien egg they discover), then, abruptly -- everything died. We all smirked at home, pretty sure we knew why.