I remember buying the separate series one set before Network decided to just release a complete series box set. I'm glad I sprung for the complete series. What I missed from the first box set are English subtitles but the complete series set added the ITC opening and ending title sequences after the episode, the WHO KILLED LAMB? episode (not a THRILLER episode but screened as part of the series), some other goodies, and the alternate version of IF IT'S A MAN, HANG UP (actually there are three different version, only two of which appear in the set). It's too bad that they weren't able to source all of the alternate footage that appeared in the ITC versions as not only was new US shot footage inserted but some original outtakes were supposed to have been used as well.
I've been trying to collect the alternate ITC versions on DVDR as well. I've got:
THE KILLER WITH TWO FACES
KILLER IN EVERY CORNER
THE NEXT VICTIM
THE DEVIL'S WEB
I have THE CARNATION KILLER (aka THE COLOUR OF BLOOD) on tape only (a rare release from the defunct big box company AIR Video).