This programme is a true masterpiece; possibly the best television series ever made by the BBC.
When I watched it as a young teenager, it was totally memorable and gripping. It is probably the only TV series of its kind from which I could recall every character and many of the plots some 25 or 30 years later. Some of the episodes toward the end of the final series were utterly gripping. The acting was outstanding; the writing fantastic, and the music and intro/credits I recall to this day with total clarity.
UDATE: Having just watched it again on DVD almost 30 years later, my view hasn't changed at all. No sense of disappointment here - this really is classic television at its very best. A clever, nuanced and absorbing series in every respect.