The most important evaluation of the 1981 BBC Radio Lord of the Rings is that I have listened to it many times - most recently over the past few weeks. There is a lot to enjoy, and that enjoyment is enhanced by repeated listening; and some aspects are absolutely excellent.
Ultimately; I do not think it possible to adapt LotR for radio in a wholly satisfactory form, due to the constraints of the medium - but this version does very well those kinds of things that radio does best; which are the small scale, inter-personal dramas of the story.
That, indeed, seems to have been a guiding principle in the highly necessary process of selection; because the script jumps rather swiftly between such scenes - compressing the exposition, travels, crowds, and battles; which probably cannot be realized on radio.
At any rate; Helm's Deep and Pelennor Fields are neither done effectively, such that Volume one ("The Fellowship of the Ring") seem (as a whole) the best done of the three books.
Given such opportunities by the script; several of the characterizations are memorable and powerful. Gandalf could not be bettered, Gollum is superb, Merry and Pippin are exactly right, Barliman Butterbur likewise. Sam is very well acted and the character developed, but I didn't find Bill Nighy's 'Mummerset' accent terribly convincing. I was not so keen on Frodo - who seemed over-emphatic in his mood changes, or Aragorn - who was inconsistent.
As seems usual in performance of LotR, the songs present a problem which is only partially solved. On the whole, I didn't find the quasi-classical music to be very appropriate or pleasing (except for the simple hobbit songs), and it was sometimes distinctly grating (especially the counter-tenor eagle, accompanied by harsh xylophone chords, whose chanting announces victory to the city of Minas Tirith - admittedly one of the lowest points of the original text).
The main weakness is that I do not think someone unfamiliar with the books would be able to follow the story - in particular I think the 'action' scenes would be confusing.
This is mostly the limitations of radio; and could only have been overcome by having long passages of narration - which would probably have spoiled the inter-personal, dramatic, scenes that are this adaptations greatest strength.
I think the version's major virtue is in its overall spirit. It comes across as a sincere and highly-motivated adaptation - produced, directed, written... put-together (where it mattered) by people* who loved Tolkien's book and were doing their very best.
(For instance, in my experience, any adaptation by Brian Sibley always provides something valuable.)
Thus I find the whole thing likeable, and feel considerable affection towards it - warts and all!
And - at its frequent best, in the scenes of conversations between major characters - the 1981 BBC Radio LotR is often variously amusing, sad, charming, frightening, and beautiful.
In sum; it enhanced my appreciation and understanding of the original, as well as being enjoyable in its own right.