Having been born some 18 years after Fabian was broadcast I have only been able to see two episodes but it strikes me as a professional and proficient attempt at a very early crime/detective drama.
The series followed dramatised cases from the real-life Inspector Fabian of Scotland Yard, and as such provided 50's audiences with something more real than purely fictional accounts such as Dixon of Dock Green. Forensic science and logic were used and the 30 minute episodes always ended with the real life Inspector Fabian recalling the case - which seems usually to have been murder.
The series is dated of course, but like watching a 50's movie one can find it entertaining. I enjoyed the two episodes that I have seen, and having been shot on film I would assume many more still exist.