I wouldn't undervalue them, Ian. You're right, after Gollancz bought out Millennium, they transfered some of the titles from this series across to the Masterworks, but initially this was their rival to the Masterworks, and while you could argue that one or two in the series don't quite make the grade, you could argue that of the Masterworks as well.
Besides, do you really believe that titles such as this don't merit reissue in such a series?
Poul Anderson -- Tau Zero,
Isaac Asimov -- The End of Eternity,
Roger Zelazny -- Isle of the Dead,
Strugatsky Brothers -- Roadside Picnic,
Robert Silverberg -- Thorns,
Octavia Butler -- Wild Seed,
John Brunner -- The Jagged Orbit
Jack Vance -- Big Planet
Ian Watson -- The Embedding
Robert Heinlein -- The Door into Summer
Hal Clement -- Mission of Gravity
Eric Frank Russell -- Wasp
Clifford D. Simak -- Way Station
John Varley -- The Ophiuchi Hotline
Walter M. Miller Jr -- The Best of (renamed Dark Benediction in the Masterworks series).
Roger Zelazny -- Damnation Alley
Ian Watson -- Miracle Visitors
Pohl & Kornbluth -- Wolfbane
Cecilia Holland -- Floating Worlds
Harry Harrison -- A Transatlantic Tunnel, Hurrah!
Pat Cadigan -- Mindplayers
... to name but a few.
As I say, the Gollancz Collectors' Series wasn't perfect, but nor is the Masterworks (too much Dick, no Asimov and no Heinlein, for starters), but both brought some wonderful, wonderful SF back into print, for which I for one am extremely grateful.