I bought that recording on LP when it first came out. The most notable thing about it was that the Mad Scene was played as composed, with glass harmonica. However, I could not get along with Sills's voice which I found shallow in tone and fluttery, a matter of taste I guess.
I have the same problem with Sills. Though she was an intelligent singer, with a keen dramatic awareness, I found the voice too pallid and fluttery for much of the repertoire she sang. The Donizetti Tudor queens suffer in that respect. They lack grandeur, especially compared to Caballe in all three, and Callas in Anna Bolena
I do quite enjoy her Giulietta in Bellini's I Capuleti e i Montecchi
(with Baker a superb Romeo) which suits her light voice better, and the voice is also well suited to Massenet's Manon, which she gives an excellent account of in her studio recording. That said, I still prefer the golden-voiced De Los Angeles in the role.
For Lucia, well it has to be Callas or early Sutherland. I also quite like Caballe's less showy rendition, which restores the original keys (actually higher for the Mad Scene, which means no stratospheric top notes).