This story has some good ideas and has its moments but overall it has too many flaws and less interesting parts to be anywhere near top quality classic Who.
The Doctor, Ian and Barbara are up to their usual well written standard and the story has very interesting aspects. The alien race The Sensorites (relatives of the Ood from 21st Century series of 'Doctor Who') are interesting as they at first seem very menacing but turn out to only have a few members of real menace with the majority being gentle and friendly. They are pretty well realised aliens, somewhat more three dimensional than your usual TV or movie alien race.
The adventure involves tension at times, trust issues and devious schemes and maintains some interest overall though it sometimes loses momentum and drags. It is good in some ways but has flaws in execution at times. Not the best adventure by classic Who standards but a solid effort within the limitations.
My Ratings:
Episode 1 - 6.5/10,
Episode 2 - 7/10,
Episode 3 - 7.5/10,
Episodes 4, 5 & 6 - 7/10.
Overall - 7/10.