Atom Heart Mother is one of the oddballs in the catalogue. I think of it as something of a transitional album. They'd always done longer tunes and shorter tunes, but the longer ones tended to be very open, spacey things with a lot of improvision, almost jam-band stuff. The shorter ones, I always think of Syd's silly British things like "Bike". But with AHM, both sides were evolving and getting more depth. The longer cuts were actually taking form, and AHM might be seen as the first really serious step towards prog.
So anyway, because of all that, it's not surprising that it divides people. There are definitely parts that don't work for me, but others parts I think are brilliant.