Well, if you didn't like Piper at the Gates of Dawn, then you probably wouldn't like A Saucerful of Secrets either (it's got a couple good tracks on it though). The time between Saucerful and Meddle was a pretty strange time for the band, sound-wise, so it definitely isn't for everyone (I'm looking at you, "Atom Heart Mother"). Other than that though, you aren't missing a whole lot.

Now I do strongly recommend you pick up a copy of "Relics", as it's got one of my all-time favorite Pink Floyd songs on it:

(stick with it, it's got a guitar solo near the end that'll melt your face off)

Edit: The first disc of Ummagumma (the one with all the live tracks) is pretty damn good, but the second disc.....ehhhhhhhhhhhhhhh.......