Agreed re: 'Ummagumma"
The 2 cuts you mentioned are all that's really worthwhile from the studio album and even those are pretty dull. And "Pompeii" basically makes the live set redundant even though the latter includes "Astronomy Domine" (just doesn't sound right without Syd IMO). It's the only classic Floyd I never listen to. But then, I think both "Saucerful" and "Atom" are extremely underrated and their best 2 albums after "Piper".
Someone here described "Obscured By Clouds" as a good album that's instantly forgotten as soon as it's over (or something to that effect) and I thought that was a fairly apt description.