Not everyone would agree (as is always the case), but I believe I have struck gold with this 1962 Mikado.
Having listened to the 1973 D'Oyly Carte, 1991 Mackerras, and 1957 Sargent versions, I found this one to be most rewarding of all. In each of the other three recordings, there is something to make the listener flinch. In the 1973 account, Katisha has problems getting up to her E's successfully; Jean Allister sings Katisha perfectly on the 1962 version. The 1991 recording is very good indeed, but Yum-Yum's G's in "So, please you, Sir, we much regret" sound funny; not so in the 1962 version. Sargent was inclined to slow tempi; Alexander Farris makes the 1962 version one of the most lively readings available.
The weakest member of the Sadler's Wells cast is probably Yum-Yum. That said, she is still a fine singer. The age of the recording (or something) puts a slight edge on her voice at times. She reminds me heavily of one of the D'Oyly Carte sopranos of the 1930's. This is no huge disappointment; she just isn't Valerie Masterson (but who is?).
Clive Revill is an excellent Ko-Ko, but for some reason, the recording team placed is "Tra la la's" on opposite speakers in "The flowers that bloom in the spring." It runs thus: "(Left speaker) The flowers that bloom in the spring, tra (right speaker) la." Such happens on every one of his concluding "la's."
The only other negative thing I can think of (beside the Mikado not attempting an evil laugh), is the sound is a bit veiled compared to other recordings. Still, it is definitely good enough, and does not alloy enjoyment.
The Iolanthe highlights maintain a similar quality in singing, though the fact that they did not (or were unable to) go the extra mile and record the entire thing is somewhat irritating. The Act I highlights are somewhat on the light side; all but two numbers of Act II are included.
Elizabeth Harwood is overly criticized in her Gilbert and Sullivan roles. She sings excellently here, though her voice is quite different from, say, Mary Sansom's on the 1960 D'Oyly Carte version. It is a joy to hear a young Heather Begg in the role of the Fairy Queen. Marjorie Thomas, soubrette star of the Sargent series, here gives a beautiful performance of Iolanthe's song and following scene (she hops up to that G-flat wonderfully).
Alexander's natural feel for G&S tempi fails him in one number on these highlights. "When I went to the bar" is taken at hyper-speed, all but robbing it of its charm. Eric Shilling, who sings The Lord Chancellor, sings a bit (I don't know how to put it) "bumpily" in places, particularly in the Nightmare Song. Just an impression.
(Note: of the songs included in the Iolanthe highlights, two lack second verses: "None shall part us," and "Soon as we may, off and away." Also, the "`Twill plunge them into grief and shame" section is omitted in "With Strephon for your foe.")
Jean Allister's Katisha, in my opinion, makes this recording worth while. Snatch it up before it vanishes indefinitely.
EDIT March 2010: I have since cooled slightly to this recording. It is indeed excellent, but I now give the title of Best Mikado to the 1957 D'Oyly Carte recording. It's available on Amazon for $2 in an acceptable mono download. Just search for "Godfrey Mikado." Happy listening!