I read Merlin's Harp a few months ago and set it aside with a vague sense of disappointment, but find that now it's sticking with me in unexpected ways. I think the main flaw in this is that Crompton didn't develop the characters very much, she simply plucked up the characters we know so well and presented them once more, although from a different viewpoint.
The language is beautifully poetic and it is a startlingly different viewpoint, although, as I said, it's the same story. She had a few nice twists, although more twists to THE story than to her story, as I didn't have any trouble seeing where they would lead.
I'd really only recommend this if you've already read a lot of other stories and versions of Arthurian Legend. It's a good addition, but not so in depth or well told that it stands on its own, without all the works that have come before.