I understand that there's always a concern about the quality of catalog releases on Blu-Ray, but Fox has gotten titles like Big Trouble In Little China, Princess Bride, Romancing The Stone and others right on; all releases that are relatively minor compared to the Alien franchise. Fox absolutely stunk in the early days, but their catalog Blu-Rays have been mostly outstanding lately. Clean film scans, no DNR, no Edge Enhancement. To top it off, Alien and Aliens are virtually guaranteed to go through a director approval process as well and I think its safe to trust Mr. Scott and Mr. Cameron.
If Paramount owned these films I'd be very worried. Fox however has earned my trust and I'll pre-order this set without concern.