Consider Phlebas is like an action movie with a few parts that could be omitted entirely (the eaters of filth) and the book would be just as good. That being said, it far and above gives you a full background in the entire culture universe so the rest of the series makes much more sense.

I actually really enjoyed Use of Weapons and think Player of Games (the ending at least) might be one of my all time top 5 favorite books I've read. Brilliant stuff, keep reading it!