Sills became probably the most famous American opera singer since Maria Callas. She was the American answer to the British Empire's Joan Sutherland (who sang most of the same repertoire). Her career followed a similar trajectory to Sutherland's too. Sutherland was singing leading roles at Covent Garden for years without actually becoming a star; she was the sort of person who got to be Aida or Violetta on nights when the company couldn't sign someone more famous, before she became an international star overnight in a Covent Garden production of Lucia Di Lammermoor. Sills was a bit better known for her work at the New York City Opera -- she got to record the title role in Douglas Moore's The Ballad of Baby Doe -- but she didn't become a big star until the 1966 production of Handel's Giulio Cesare.
RCA made a recording based on that production, which is worth hearing for Sills and the fun of hearing all the singers perform Handel as just another opera, without the preciousness and carefulness that sometimes infects today's Historically Informed Performances. (The recording is too heavily-cut and unevenly cast to be a first choice for the opera, though.) Conrad L. Osborne's review of the recording is available online, and I can do no better than to quote from his review of Sills's work:
For most of us who attended the production Beverly Sills’s Cleopatra meant a “discovery” almost as startling as that of Joan Sutherland’s Lucia nearly nine years ago. Through the past decade, this singer had shown herself an excellent artist in a variety of roles, and in such diverse assignments as Philine and Baby Doe had hinted at the qualities finally displayed in her Cleopatra. Still. I don’t think any of us quite expected the classic exhibition of vocal control, agility, freedom, and command she gave us that evening (and on other occasions since). The singing was reinforced with splendid stylistic instinct, grace of movement, and communicative feminine warmth which would almost have been enough by itself-the effect was one of sheer magic. Cleopatra has five major arias, and five times in the course of the evening everything in the theatre was suspended on a fragile (but strong) thread of floating, silvery tone, on the proverbial string of pearls that every singer wants to make of a run, and the most gorgeous of all trills. It was a feast for sore ears, and brought with it the recognition that if the prevailing standard in Handel’s day was something like this, the willingness of audiences to sit and listen to entire evenings of arias festooned with inventions is entirely understandable-a sensuous indulgence of an almost shameful order.
The recording. happily, has found Miss Sills in excellent form, and has captured a healthy portion of the purely vocal side of the magic. It is the last three arias that really take the breath away; the first two are very fine, but there are traces of unsettlement when she sustains tones around the top of the staff. and there are other “Piangeròs” on records that offer healthy competition. But “V’adoro, pupille” is a really melting piece of vocal seduction; “Se pieta,” with its beautiful flights of trills, high suspensions, and beautifully floated harmonic turns is heartachingly lovely; and the “Da tempeste” simply takes off into the ionosphere. A measure of this singing is that Miss Sills trills a hundred times if she trills once, and one never tires of the lovely, truly birdlike sound.