Fifty-one years later, how do we appraise “Let It Be,” the Beatles’ swan song, the document of their breakup, the one that the bandmembers themselves initially disliked so much that Paul McCartney took legal action and John Lennon dubbed it a salvage job from “the shittiest load of badly recorded shit — and with a lousy feeling to it — ever”?

Of course, the men doth protest too much: The group’s high standards guaranteed that there is no such thing as a bad Beatles album, but “Let It Be” is far from their best work. Intended as a rough, back-to-the-roots “art as it happens” film-and-album document of the Beatles’ creative process, it ended up being a requiem. Many factors contributed to the album’s often dispirited vibe: Except for Ringo, the bandmembers had been together since their early teens — residing in a nearly unprecedented fishbowl of fame for the previous five years — and tempers were wearing thin, a situation exacerbated by the fact that for this project, they were trying to find creative inspiration first thing in the morning during a typically miserable English winter while their every move was recorded by a film crew.

The group rejected two early versions of the album and Lennon finally gave the tapes to “Wall of Sound” producer Phil Spector, who sonically overhauled — some say sonically mauled — the entire thing, adding an orchestra and/or choir to several songs (hence the legal letter from McCartney). Even though it wasn’t actually the final Beatles album to be recorded — work began on “Abbey Road” several weeks after this — it was the last one to be released. Ultimately, despite several classic songs, “Let It Be” resides in the lower echelon of the Fab Four canon, along with the rushed “Beatles for Sale” and their fledgling 1963 debut.

Still, the Beatles recorded hundreds of hours of music during the long, plodding January 1969 sessions for this album, and although archivists have dug into the well before — exhuming bits for the “Beatles Anthology” series and the McCartney-helmed, de-Spectored “Let It Be… Naked” revisionist album in 2003 — they’ve given it the full museum treatment here, producing not just this lavish, 6-CD boxed set but also the six-hour “Get Back” documentary (airing on Disney+ next month) and a coffee-table book that includes hundreds of photos and transcriptions of apparently every interesting bit of dialogue from the film reels (there’s a different, exhaustively detailed hardcover book accompanying this boxed set). Thus, it’s surprising that amid those six discs, just two-and-a-half contain genuinely unreleased material, along with remastered and Blu-ray versions of the original album as well as the first of the two earlier, rejected versions of it (which has been circulated on bootlegs for decades).

The Beatles’ “Let It Be Special Edition” super deluxe edition with 5 CDs and one BluRay Courtesy Apple

While the relatively small amount of new material was met with howls of outrage from completist-leaning fans when the tracklist was first announced, it’s actually a relief — a refreshingly concise distillation of the best outtakes from the weeks of sessions, which were essentially rehearsals. And anyone who’s watched a band rehearse for more than 15 minutes knows that with the possible exception of Prince (who usually rehearsed as if he were onstage at Madison Square Garden on New Year’s Eve), most of the time they are excruciatingly dull. And as the countless hours of bootlegs from these sessions that have emerged over the decades prove, that’s even true of the Beatles.

So herein is the best of the rest: a full disc called “Apple Sessions” of loose, alternate takes of songs from the album, with the group chatting and joking around, vamping on “Maggie Mae” and the Everly Brothers’ “Wake Up Little Susie,” McCartney throwing in a snippet of “Please Please Me,” the group’s first No. 1 single, before launching into “Let It Be.”

Much more interesting is the “Get Back — Rehearsals and Apple Jams” disc, where we hear rough, embryonic versions of songs that later turned up on “Abbey Road” and even the solo albums (Lennon’s “Gimme Some Truth” and Harrison’s “All Things Must Pass”). Most arresting is the rehearsal of Harrison’s “Something,” where he says he just can’t get the line that ultimately became “Attracts me like no other lover.” “Just say whatever comes into your head,” Lennon suggests. “’Attracts me like a cauliflower.’”

There’s also the rejected version of the album, produced and assembled by Glyn Johns. Although he was one of the greatest producers of the rock era (his work with the Rolling Stones, the Who and virtually every great British artist of the time is legendary), it’s easy to see why the Beatles didn’t want to release the album in this form: While it presents an interesting alternate view of the album, it’s shambolic and at times extremely sloppy; during a loose jam entitled “Rocker,” the group sings the ‘50s classic “Save the Last Dance for Me” painfully off-key. Throw in another disc containing just four alternate mixes, and that’s the lot.

While the collectors’ instinct might be to feel short-changed (why do a four-track EP when they could have filled the disc with 20 more songs?), the rough nature of the previously unreleased material here doesn’t exactly leave one thirsting for more. Anyone who is can check out the hours and hours of outtakes on bootlegs (there was even a 17-CD set called “Thirty Days,” which probably feels like it goes on for 30 days) or YouTube — not to mention the six-hour long “Get Back” film. The only major tracks we can think of that might be missing are Lennon’s languid take on “Mailman, Bring Me No More Blues” (which is available on the 1990s “Beatles Anthology”) and this rough version of “Get Back” sung by Lennon instead of McCartney.

Even half a century later, the group’s sterling quality control remains, and if this lavish, multi-part treatment of the Beatles’ swan song is truly the last dance, they’ve made the most of it. And if we too doth complain too much, for context, let’s paraphrase Paul McCartney’s response to criticism of the “White Album” in the 1990s “Beatles Anthology” series: “It was great, it sold, it’s the bloody Beatles — shut up!”