Some things to consider if you're coming to this new:
It's the fifth full-length Pink Floyd album released in a little over three years, but if we're being entirely completist here, there's another whole album's worth of material between 'Ummagumma' and this. At the end of 1969, with the tour to support 'Ummagumma' in full flow, the band were asked to contribute to another film soundtrack; Antonioni's less than successful 'Zabriskie Point'. Unlike 'More', where they were pretty much left to their own devices to score the whole film, Antonioni had some very clear ideas of what he wanted to fill gaps in an existing score. This didn't work particularly well - the impression you get is of two sets of egos never quite being comfortable enough to compromise on their vision. At one point, Floyd produced a full score, but Antonioni knew what he wanted, and in the end, a few pieces of Pink Floyd made it to the final version, including a reworked 'Careful With That Axe, Eugene' over the multiple explosions which provide the climax to the film.
You can hear most of the music which was made for 'Zabriskie Point' here; some of it ends up on 'Dark Side of the Moon'…
In addition, Gilmour and Wright (I think Roger Waters had given up by this point) worked on Syd Barrett's second (and final) album, 'Barrett' while they were making this. You can hear that here:
I think it's a terrific piece of Syd music, but he was clearly not coping with anything by this point, and he basically retired from life after this.
Partly because of that, this album, to me, marks the point at which the band stops being 'Syd Barrett's Pink Floyd', and start to carve out their own sound as 'Pink Floyd'. I think it's a useful distinction; although they don't quite get the mix right, they are a lot closer to what people think of as 'classic' Floyd here.
Pink Floyd albums were never really made in isolation; much of their work is tied up with collaborators, and 'Atom Heart Mother' is really an album by Pink Floyd and Ron Geesin, who produced bits of it, arranged the orchestral pieces (not always successfully, it has to be said), and had a significant input into how the final album sounded.
The final sound, however, is a little compromised. Not only was it designed to be issued in quadrophonic sound (it was the Next Big Thing for most of the Seventies without ever really taking off), but it was recorded on brand new eight-track tape, which EMI had declared they didn't want chopped up, edited or otherwise mucked about with. This meant that Mason and Waters had to record the entire rhythm track for the title track in one take. Mason isn't the world's greatest drummer; if it sounds like the tempo lurches about in places, it's probably down to him. Also, recording over this track with an orchestra proved tricky, as the backing track kept leaking into the recording - if it sounds a bit weedy in places, that's why.
It also proved difficult to get the orchestra to take any of this seriously, and it wasn't until the choir arrived, complete with their choirmaster who was an actual, properly qualified orchestral conductor, that anything got done to anyone's satisfaction.
The other principal collaborators on this are, of course, Storm Thorgerson and Aubrey Powell, better known as Hipgnosis. This wasn't Hipgnosis' first Floyd album cover, but it was the first for which the only appropriate adjective is 'iconic'.
Thorgerson and Powell were friends of Syd and David from Cambridge who were art students - they did the cover for 'Saucerful of Secrets' partly because they were cheap, having access to their University darkroom and equipment at the time. They subsequently became entwined with the visual part of the Pink Floyd experience, and this is the first full expression of what they could do - no clues on the cover to the band or album title; they fulfilled the band's request for a cover without any of that psychedelic or space-rock stuff. Legend has it that the cow was literally the first thing Thorgerson photographed as he looked for inspiration. It bears no relation to the title (which was apparently a headline from a newspaper about a woman who had given birth after having a pacemaker fitted), and the cow is only related to the music because some of the parts of the title track were named after the picture had been agreed upon.
The cover, however, made 'Atom Heart Mother' one of those albums it was cool to own - whether or not you actually played it.
So, is it any good?
It is the first album on which you hear the elements coming together to make the Pink Floyd sound; I could argue that this doesn't happen enough. The title track is a triumph of perseverance - it is probably closer to the original intent that it has any right to be, given the chaos of the recording sessions. It's a proper piece of music, with themes and recapitulations and everything - the reason it doesn't overstay its welcome at 24 minutes long is that it's very well written; it takes you on its journey without getting sidetracked too often, and the choral element is simply gorgeous.
The other side goes back to 'everybody take one song each', and is weaker because of that. Waters whinges on for four and a half minutes about what a wanker he is, but it's not his fault that all these women keep throwing themselves at him, and his wife should just accept that he's a bit of a wanker; life would be much easier. Extra marks for use of the subjunctive, though.
Wright's evocation of when Syd was still in the band is fun, but hardly essential.
Gilmour apparently had to be locked in the studio until he came up with 'Fat Old Sun'; it's a great song, but if he's struggling with the songwriting,maybe everyone could pitch in and help, rather than giving him detention. We've already established that Pink Floyd sound better when everyone is working together; this insistence on everyone contributing something is getting a bit silly.
Mason doesn't really write anything for his bit - he apparently 'arranged' 'Alan's Psychedelic Breakfast' - again, there are parts when this works, but the use of 'voices off' needs to be refined more than a little - they are still too interested in the rhythmic possibilities of dripping taps to see the whole picture. When it works, it works, though.
Overall, I like this more than the people who made it do; I think its reputation is driven as much by the cover as by the music, but it's still moving everything forward, and what came after it wouldn't have been possible if they hadn't made this first.