The key to riff based music to be tolerable is to vary the surrounding texture so that even though you have consistent riff the music is always dynamically changing. I think David Bowie succeeds with that
Also his vocal melodies kinda sucks but thankfully this is compensated by sme really good instrumental melodies supplied by the saxophone and guitarist. They supply the tunefulness that is a absense from the vocals
I probably give it a 11/15 album, but I can see how someone who doesn't have a massive bias against riff based music could see it as an all time classic album. Although saying that it's nowhere near Can Tago Mago in the best riff based album of all time.
I gave a track by track thoughts here on John Mc board